The sun is subject to terrific hurricanes and cyclones, as well as explosions, casting up jets to the height of 200,000 miles. In the early days of spectroscopy these protuberances could only be seen at a time of a total solar ellipse, and astronomers made long journeys to distant parts of the earth to be in line of totality. Now all is changed. Images of the sun are thrown into the observatory by an ingenious instrument run by clockwork, and called a heliostat. This is set on the sun at such an angle as to throw the solar image into the objective of the telescope placed horizontally in a darkened observatory, and the pendulum ball set in motion, when it will follow the sun without moving its image, all day if desired. At the eye end of the telescope is attached the spectroscope and the micrometer, and the whole set of instruments so adjusted that just the edge of the sun is seen, making a half spectrum. The other half of the spectroscope projects above the solar limb, and is dark, so if an explosion throws up liquid jets, or flames of hydrogen, the astronomer at once sees them and with the micrometer measures their height before they have time to fall. And the spectrum at once tells what the jets are composed of, whether hydrogen, gaseous iron, calcium, or anything else. Prof. C. A. Young saw a jet of hydrogen ascend a distance of 200,000 miles, measured its height, noted its spectrum and timed its ascent by a chronometer all at once, and was astonished to find the velocity one hundred and sixty miles per second—eight times faster than the earth flies on its orbit. By these improvements solar hurricanes, whirlpools, and explosions can be seen from any physical observatory on clear days.

The slit of the spectroscope can be moved anywhere on the disk of the sun; so that if the observer sees a tornado begin, he moves the slit along with it, measures the length of its tract and velocity. With the telescope, micrometer, heliostat, and spectroscope came desire for more complex instruments, resulting in the invention of the photoheliograph, invoking the aid of photography to make permanent the results of these exciting researches. This mechanism consists of an excessively sensitive plate, adjusted in the solar focus of the telespectroscope. In front of the plate in the camera is a screen attached to a spring, and held closed by a cord. The eye is applied to the spectroscopic end of the complex arrangement to watch the development of solar hurricanes.

Finally an appalling outburst occurs; the flames leap higher and higher, torn into a thousand shreds, presenting a scene that language is powerless to describe. When the display is at the height of its magnificence, the astronomer cuts the cord; the slide makes an exposure of one-three thousandth part of a second, and an accurate photograph is taken. The storm all in rapid motion is petrified on the plate; everything is distinct, all the surging billows of fire, boilings, and turbulence are rendered motionless with the velocity of lightning.

At Meudon, in France, M. Janssen takes these instantaneous photographs of the sun, thirty inches in diameter, and afterward enlarges them to ten feet; showing scenes of fiery desolation that appalls the human imagination. (See address of Vice President Langley, A. A. A. S., Proceedings Saratoga Meeting, p. 56.) This huge photograph can be viewed in detail with a small telescope and micrometer, and the crests of solar waves measured. Many of these billows of fire are in dimensions every way equal in size to the State of Illinois. Binary stars are photographed so that in time to come they can be retaken, when if they have moved, the precise amount can be measured.

Another instrument is the telepolariscope, to be attached to a telescope. It tells whether any luminous body sends us its own, or reflected light. Only one comet bright enough to be examined has appeared since its perfection. This was Coggia's, and was found to reflect solar from the tail, and to radiate its own light from the nucleus.

Still another intricate instrument is in use, the thermograph, that utilizes the heat rays from the sun, instead of the light. It takes pictures by heat; in other words, it sees in the dark; brings invisible things to the eye of man, and is used in astronomical and physical researches wherein undulations and radiations are concerned. And now comes the magnetometer, to measure the amount of magnetism that reaches the earth from the sun. It points to zero when the magnetic forces of the earth are in equilibrium, but let a magnetic storm occur anywhere in the world and the pointer will move by invisible power. It detects a close relation between the magnetism of the earth and sun. The needle is deflected every time a solar disturbance takes place. At Kew, England, an astronomer was viewing the sun with a telescope and observed a tongue of flame dart across a spot whose diameter was thirty-three thousand seven hundred miles. The magnetometer was violently agitated at once, showing that whatever magnetism may be, its influence traversed the distance of the sun with a velocity greater than that of light.

Not less remarkable is the new instrument, the thermal balance, devised by Prof. S. P. Langley, Pittsburgh. It will measure the one-fifty-thousandth part of a degree of heat, and consists of strips of platinum one-thirty-second of an inch wide and one-fourth of an inch long; and so thin that it requires fifty to equal the thickness of tissue paper, placed in the circuit of electricity running to a galvanometer. "When mounted in a reflected telescope it will record the heat from the body of a man or other animal in an adjoining field, and can do so at great distances. It will do this equally well at night, and may be said, in a certain sense, to give the power of seeing in the dark." (Science, issue of Jan. 8,1881, p. 12.) It is expected to reveal great facts concerning the heat of the stars.

Indeed, the thermopile in the hands of Lockyer has already made palpable the heat of the fixed stars. He placed the little detective in the focus of a telescope and turned it on Arcturus. "The result was this, that the heat received from Arcturus, when at an altitude of 55 deg., was found to be just equal to that received from a cube of boiling water, three inches across each side, at the distance of four hundred yards; and the heat from Vega is equal to that from the same cube at six hundred yards." (Lockyer's Star Gazing, p. 385.) Thus that inscrutable mode of force heat traverses the depths of space, reaches the earth, and turns the delicate balance of the thermopile. Another discovery was made with the spectroscope; thus, if a boat moves up a river, it will meet more waves than will strike it if going down stream. Light is the undulation of waves; hence if the spectroscope is set on a star that is approaching the earth, more waves will enter than if set on a receding star, which fact is known by displacement of lines in the spectroscope from normal positions. It is found that many fixed stars are approaching, while others are moving away from the solar system.

We cannot note the researches of Edison, Lockyer, or Tyndall, nor of Crookes, who has seemingly reached the molecules whence the universe is composed.

The modern observatory is a labyrinth of sensitive instruments; and when any disturbance takes place in nature, in heat, light, magnetism, or like modes of force, the apparatus note and record them.

Men are by no means satisfied. Insatiable thirst to know more is developing into a fever of unrest; they are wandering beyond the limits of the known, every day a little farther. They survey space, and interrogate the infinite; measure the atom of hydrogen and weigh suns. Man takes no rest, and neither will he until he shall have found his own place in the chain of nature.—Kansas Review.

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THE FUTURE DEVELOPMENT OF ELECTRICAL APPLIANCES.

Prof. J. Perry lately delivered a lecture on this subject at the Society of Arts, London, which contains in an epitomized form the salient points of the hopes and fears of the more sanguine spirits of the electrical world. Prof. Perry is one of the two professors who have been dubbed the "Japanese Twins," and whose insatiate love of work induced one of our most celebrated men of science to say that they caused the center of experimental research to tend toward Tokyo instead of London. Professors Ayrton and Perry have for some time been again resident in England, but it is evident that they did not leave any of their energy in Japan, for those who know them intimately, know that they are pursuing numerous original investigations, and that so soon as one is finished, another is commenced. It would have been difficult then to have found an abler exponent of the future of electricity.

Prof. Perry, after referring to what might have been said of the great things physical science has done for humanity, plunged into his subject. The work to be done was vast, and the workers altogether out of proportion to the task.

The methods of measurement of electricity are not generally understood. Perhaps when electricity is supplied to every house in the city at a certain price per horse power, and is used by private individuals for many different purposes, this ignorance will disappear. Electrical energy is obtained in various ways, but the generators get heated; and one great object of inventors is to obtain from machines as much as possible electrical energy of the energy in the first place supplied to such machine. The lecturer called particular attention to the difference between electricity and electrical energy, and attempted to drive home the fundamental conceptions of electrical science by the analogies derivable from hydraulics. A miller speaks not only of quantity of water, but also of head of water. The statement then of quantity of electricity is insufficient, except we know the electrical property analogous to head of water, and which is termed electrical potential. A small quantity of electricity of high potential is similar to a small quantity of water at high level. The analogies between water and electricity were collected in the form of a table shown on a wall sheet as follows:

We Want to Use Water. We Want to Use Electricity.

1. Steam pump burns coal, 1. Generator burns zinc, or and lifts water to a higher uses mechanical power, and level. lifts electricity to a higher level or potential.

2. Energy available is 2. Energy available is amount of water lifted x amount of electricity x difference difference of level. of potential.

3. If we let all the water 3. If we let all the electricity flow away through channel flow through a wire from one to lower level without doing screw of our generator to the work, its energy is all other without doing work, all converted into heat because the electrical energy is of frictional resistance of converted into heat because of pipe or channel. resistance of wire.

4. If we let water work a 4. If we let our electricity hoist as well as flow through work a machine as well as channels, less water flows flow through wires, less flows than before, less power is than before, less power is wasted in friction. wasted through the resistance of the wire.

5. However long and narrow 5. However long and thin may be the channels, the wires may be, electricity water maybe brought from may be brought from any distance distance, however great, however great, to give to give out almost all its out almost all its original original energy to a hoist. energy to a machine. This requires This requires a great head a great difference of and small quantity of water. potentials and a small current.

The difference between potential and electro-motive force was explained thus: "difference of potential" is analogous with "difference of pressure" or "head" of water, howsoever produced; whereas electromotive force is analogous with the difference of pressure before and behind a slowly moving piston of the pump employed by an unfortunate miller to produce his water supply. Electricians have very definite ideas upon the subject they are working at, and especial attention is paid to the measurements on which their work depends. Examples of these measurements were shown by the following tables on wall sheets:

ELECTRICAL MAGNITUDES (SOME RATHER APPROXIMATE).

Resistance of One yard of copper wire, one-eighth of an inch diameter...............................0.002 ohms. One mile ordinary iron telegraph wire, .........10 to 20 " Some of our selenium cells ............. 40 to 1,000,000 " A good telegraph insulator ........... 4,000,000,000,000 "

Electro-motive force of A pair of copper-iron junctions at a difference of temperature of 1 deg. Fah......... =0.0000 volt. Contact of zinc and copper ..................... =0.75 " One Daniell's cell ............................. =1.1 " Mr. Latimer Clark's standard cell .............. =1.45 " One of Dr. De la Hue's batteries ...... =11,000 " Lightning flashes probably many millions of volts.

Current measured by us in some experiments:

Using electrometer....... = almost infinitely small currents. Using delicate galvanometer =0.00,000,000,040 weber. Current received from Atlantic cable, when 25 words per minute are being sent ................ = 0.000,001 weber Current in ordinary land telegraph lines ......................... = 0.003 weber Current from dynamo machine.... = 5 to 100 weber

In any circuit, current in webers = electro-motive force in volts / resistance in ohms.

RATE OF PRODUCTION OF HEAT, CALCULATED IN THE SHAPE OF HORSE-POWER.

In the whole of a circuit=current in webers x electro-motive force in volts / 746. In any part of circuit=current in webers x difference of potential at the two ends of the part of the circuit in question / 746. Or, =square of current in webers x resistance of the part in ohms / 746.

If there are a number of generators of electricity in a circuit, whose electromotive forces in volts are E1, E2, etc., and if there are also opposing electro-motive forces. F1, F2, etc., volts, and if C is the current in webers, R the whole resistance of the current in ohms, P the total horse-power taken at the generators, Q the total horse-power converted into some other form of energy, and given out at the places where there are opposing electro-motive forces, H the total horse-power wasted in heat, because of resistance, then:

(E1+E2+etc.)-(F1+F2+etc.) C = ——————————————- R

[TEX: C = frac{(E1+E2+ ext{etc.})-(F1+F2+ ext{etc.})}{R}]

C C P = —-((E1+E2+etc.); Q = —-(F1+F2+etc.) 746 746

[TEX: frac{C}{746}(E1+E2+ ext{etc.}); Q = frac{C}{746}(F1+F2+ ext{etc.})]

C squared R H = ——- . 746

[TEX: H = frac{C^2 R}{746}.]

The lifting power of an electro-magnet of given volume is proportional to the heat generated against resistance in the wire of the magnet.

The future of many electrical appliances depends on how general is the public comprehension of the lessons taught by these wall sheets. If a few capitalists in London would only spend a few days in learning thoroughly what these mean, electrical appliances of a very distant future would date from a few months hence.

A number of experiments were shown, in some of which electrical energy was converted into heat, in others into sound, in others into work. At this part of the lecture reference was made to the work of Prof. Ayrton and his pupils at Cowper street (City and Guilds of London Institute Classes). They measure (1) the gas consumed by the engine, (2) the horse-power given to the dynamo machine, (3) the current in the circuit in webers, and (4) the resistance of the circuit. Thus exact calculations can now be made as to the horse power expended in any part of the circuit, and the light given out in any given period by an electric lamp. The dynamometers used in these measurements were described, but at present, in some cases, the description given is for various reasons incomplete, so that we shall take a future opportunity of writing of these instruments. To measure the light a photometer, constructed by Profs. Ayrton and Perry, is used, which obviates the necessity of large rooms, and enables the operator to give the intensity in a very short period of time. A number of measurements of the illuminating power of an electric lamp were rapidly made during the lecture with this photometer. By means of a small dynamo machine, driven by an electric current generated in the Adelphi arches, a ventilator, a sewing machine, a lathe, etc., were driven; in the latter a piece of wood was turned. "What," said the lecturer, "do these examples show you?" "They show that if I have a steam-engine in my back yard I can transmit power to various machines in my house, but if you measured the power given to these machines you would find it to be less than half of what the engine driving the outside electrical machine gives out. Further, when we wanted to think of heating of buildings and the boiling of water, it was all very well to speak of the conversion of electrical energy into heat, but now we find that not only do the two electrical machines get heated and give out heat, but heat is given out by our connecting wires. We have then to consider our most important question. Electrical energy can be transmitted to a distance, and even to many thousands of miles, but can it be transformed at the distant place into mechanical or any other required form of energy, nearly equal in amount to what was supplied? Unfortunately, I must say that hitherto the practical answer made to us by existing machines is, 'No;' there is always a great waste due to the heat spoken of above. But, fortunately, we have faith in the measurements, of which I have already spoken, in the facts given us by Joule's experiments and formulated in ways we can understand. And these facts tell us that in electric machines of the future, and in their connecting wires, there will be little heating, and therefore little loss. We shall, I believe, at no distant date, have great central stations, possibly situated at the bottom of coal-pits where enormous steam engines will drive enormous electric machines. We shall have wires laid along every street, tapped into every house, as gas-pipes are at present; we shall have the quantity of electricity used in each house registered, as gas is at present, and it will be passed through little electric machines to drive machinery, to produce ventilation, to replace stoves and fires, to work apple-parers and mangles and barbers' brushes, among other things, as well as to give everybody an electric light."

It is possible, as Prof. Ayrton first showed in his Sheffield lecture, that electrical energy can be transmitted through long distances by means of small wires, and that the opinion that wires of enormous thickness would be required is erroneous. The desideratum required was good insulation. He also showed that, instead of a limiting efficiency of 50 per cent., the only thing preventing our receiving the whole of our power was the mechanical friction which occurs in the machines. He showed, in fact, how to get rid of electrical friction. A machine at Niagara receives mechanical power, and generates electricity. Call this the generator. Let there be Wires to another electric machine in New York, which will receive electricity, and give out mechanical work. Now this machine, which may be called the motor, produces a back electromotive force, and the mechanical power given out is proportional to the back electromotive force multiplied into the current. The current, which is, of course, the same at Niagara as at New York, is proportional to the difference of the two electromotive forces, and the heat wasted is proportional to the square of the current. You see, from the last table, that we have the simple proportion: power utilized is to power wasted, as the back electromotive force of the motor is to the difference between electromotive forces of generator and motor. This reason is very shortly and yet very exactly given as follows:

Let electromotive force of generator be E; of motor F. Let total resistance of circuit be R. Then if we call P the horse-power received by the generator at Niagara, Q, the horse-power given out by motor at New York, that is, utilized; H, the horse-power wasted as heat in machines and circuit; C, the current flowing through the circuit:

C=(E-F) / R

P=E(E-F) / (746 R)

Q=F(E-F) / (746 R)

H=(E-F)_2 / (746 R)

Q:H::F:E-F

The water analogy was again called into play in the shape of a model for the better demonstration of the problem. The defects in existing electric machines and the means of increasing the E.M.F. were discussed, the conclusions pointing to the future use of very large machines and very high velocities. The future of telephonic communication received a passing remark, and attention called to the future of electric railways. The small experiments of Siemens have determined the ultimate success of this kind of railway. Their introduction is merely a question of time and capital. The first cost of electric railways would be smaller than that of steam railways; the working expenses would also be reduced. The rails would be lighter, the rolling stock lighter, the bridges and viaducts less costly, and in the underground railways the atmosphere would not be vitiated.

"About two years ago, it struck Professor Ayrton and myself, when thinking how very faint musical sounds are heard distinctly from the telephone, in spite of loud noises in the neighborhood, that there was an application of this principle of recurrent effects of far more practical importance than any other, namely, in the use of musical notes for coast warnings in thick weather. You will say that fog bells and horns are an old story, and that they have not been particularly successful, since in some states of the weather they are audible, in others not.

"Now, it seems to be forgotten by everybody that there is a medium of communicating with a distant ship, namely, the water, which is not at all influenced by changes in the weather. At some twenty or thirty feet below the surface there is exceedingly little disturbance of the water, although there may be large waves at the surface. Suppose a large water-siren like this—experiment shown—is working at as great a depth as is available, off a dangerous coast, the sound it gives out is transmitted so as to be heard at exceedingly great distances by an ear pressed against a strip of wood or metal dipping into the water. If the strip is connected with a much larger wooden or metallic surface in the water the sound is heard much more distinctly. Now, the sides of a ship form a very large collecting surface, and at the distance of several miles from such a water siren as might be constructed, we feel quite sure that, above the noise of engines and flapping sails, above the far more troublesome noise of waves striking the ship's side, the musical note of the distant siren would be heard, giving warning of a dangerous neighborhood. In considering this problem, you must remember that Messrs. Colladon and Sturn heard distinctly the sound of a bell struck underwater at the distance of nearly nine miles, the sound being communicated by the water of Lake Geneva."

The next portion of the lecture discussed the great value of a rapid recurrence of effects, the obtaining of sound by means of a rapid intermission of light rays on selenium joined up in an electric circuit being instanced as an example. Then recent experiments on the refractive power of ebonite were detailed—the rough results tending to give greater weight to Clerk-Maxwell's electro-magnetic theory of light. The index of refraction of ebonite was found by Profs. Ayrton and Perry to be roughly 1.7. Clerk-Maxwell's theory requires that the square of this number should be equal to the electric specific inductive capacity of the substance. For ebonite this electric constant varies from 2.2 to 3.5 for different specimens, the mean of which is almost exactly equal to the square of 1.7.

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RESEARCHES ON THE RADIANT MATTER OF CROOKES AND THE MECHANICAL THEORY OF ELECTRICITY.

By DR. W. F. GINTL, abstracted by DR. VON GERICHTEN.

The author discusses the question whether, according to the experiments of Crookes, the assumption of an especial fourth state of aggregation is necessary, or whether the facts may be satisfactorily explained without such hypothesis? He shows that the latter alternative is possible with the aid of a mechanical theory of electricity. If the radiant matter produced in the vacuum is a phenomenon sui generis, produced by the action of electricity and heat upon the molecules of gas remaining in the receiver, it is, in the first place, doubtful to apply to it the conception of an aggregate condition. The author considers it impossible to form a clear understanding of the phenomena in accordance with the theory of Crookes, or to find in the facts any evidence of the existence of radiant matter. An explanation of the latter phenomenon is thus given: Particles become separated from the surface of the substance of the negative pole, they are repelled, and they move away from the pole with a speed resulting from the antagonistic forces in a parallel and rectilinear direction, preserving their speed and their initial path so long as they do not meet with obstacles which influence their movement. At a certain density of the gases present in the exhausted space, these particles, in consequence of the impact of gaseous molecules more or less opposed to their direction of movement, lose their velocity after traveling a short distance and soon come to rest. The more dilute the gas the smaller is the number of the impacts of the gaseous molecules encountering the molecules of the poles, and at a certain degree of dilution the repelled polar particles will be able to traverse the space open to them without any essential alteration in their speed, the small number of the existing gaseous molecules being no longer able to retard the molecules of the polar no their journey through the apparatus. The luminous phenomena of the Geissler tubes the author supposes to be produced by the intense blows which the gaseous molecules receive from the polar molecules flying rapidly through the apparatus. The intensity of the luminous phenomena will naturally decrease with the number of the photophorous particles occupying the space. Accordingly in the experiments of Crookes, on continued rarefaction of the gas, a condition was reached where a display of light is no longer perceptible, or can be made visible merely by the aid of fluorescent bodies. A condition may also appear, as is shown by Crookes' experiment, with the metallic plate intercalated as negative pole in the middle of. a Geissler tube, with the positive poles at the ends. In this case the gaseous molecules are, so to speak, driven away by the polar particles endowed with an equal initial velocity, till at a certain distance from the pole the mass of the gaseous molecules and their speed become so great that a luminous display begins. In an analogous manner the author explains the phenomena of phosphorescence which Crookes' elicits by the action of his radiant matter. In like manner the thermic and the mechanical effects are most simply explained, according to the expression selected by Crookes himself, as the results of a "continued molecular bombardment." The attraction of the so called radiant matter, regarded as a stream of metallic particles by the magnet, will not appear surprising.

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ECONOMY OF THE ELECTRIC LIGHT.

Mr. W. H. Preece writes to the Journal of Arts as follows:

At the South Kensington Museum, very careful observations have been made on the relative cost of the two systems, i. e., gas and electricity. The court lighted is that known as the "Lord President's" (or the Loan) Court. It is 138 feet long by 114 feet wide, and has an average height of about 42 feet. It is divided down the middle lengthwise by a central gallery. There are cloisters all around it on the ground floor, and the walls above are decorated in such a way that they do not assist in the reflection or diffusion of the light. The absence of a ceiling—the court being sky-lighted—is to some extent compensated for by drawing the blinds under the sky-lights.

The experiments commenced about twelve months ago, with eight lamps only on one side of the court. The system was that of Brush. The dynamo machine was driven by an eight horse-power Otto gas engine, supplied by Messrs. Crossley. The comparison with the gas was so much in favor of electricity, and the success of the experiment so encouraging, that it was determined to light up the whole court.

The gas engine, which was not powerful enough, was replaced by a 14-horse power "semi-portable" steam engine, by Ransomes & Co., of Ipswich—an engine of sufficient power to drive double the required number of lights. The dynamo machine is a No. 7 Brush. There are sixteen lamps in all—eight on each side of the court. The machine has given no trouble whatever, and it has, as yet, shown no signs of wear. The lamps were not all good, and it was found that they required careful adjustment, but when once they were got to go right they continued to do so, and have, up to the present, shown no signs of deterioration, although the time during which they have been in operation is nine months.

The first outlay has been as follows:

Engine and fixing, including shafting and belting................................ L420 Dynamo machine......................... 400 Lamps, apparatus, and conducting wire . 384 ——— L1,204

The cost of working has been, from June 22, to December 31, during which period the lights were going on 87 nights for a total time of 359 hours:

L s. d. Carbons............................... 18 9 0 Oil, etc.............................. 4 11 6 Coal.................................. 11 14 0 Wages................................. 34 7 6 ————— L69 2 0

being at the rate of 3s. 10d. per hour of light.

Now, the consumption of gas in the court would have been 4,800 cubic feet per hour, which, at 3s. 4d. per 1,000 cubic feet, would amount to 16s. per hour, thus showing a saving of working expenses of 12s. 2d. per hour, or, since the museum is lit up for 700 hours every year, a total saving at the rate of L426 per annum.

In estimating the cost as applied to this court, only half the cost of the engine should be taken, for a second dynamo machine has lately been added to light up some of the picture galleries, and the "Life" room of the Art School. The capital outlay should, therefore, be L994. In making a fair estimate of the annual cost, we should also allow something for percentage on capital, and something for wear and tear. Take—

L s. 5 per cent, on the capital............................. 49 10 5 per cent, for wear and tear of electrical apparatus.. 39 0 5 per cent, for depreciation of engines, etc........... 21 0 ———- Total.......... L109 10

leaving a handsome balance to the good of L316 10s. as against gas. The results of the working, both practically and financially, have proved to be, at South Kensington, a decided success.

I am indebted to Colonel Festing, R.E., who has charge of the lighting, for these details.

The same comparison cannot be made at the British Museum, for no gas was used in the reading-room before the introduction of the electric light, but the cost of lighting has proved to be 5s. 6d. per hour—at least one-third of that which would be required for gas. The system in use at the Museum is Siemens', the engine being by Wallis and Steevens, of Basingstoke.

"An excellent example of economic electric lighting, is that of Messrs. Henry Tate & Sons, sugar refinery, Silvertown. A small Tangye engine, placed under the supervision of the driver of a large engine of the works, drives an 'A' size 'Gramme' machine, which feeds a 'Crompton' 'E' lamp. This is hung at a height of about 12 feet from the ground in a single story shed, about 80 feet long, and 50 feet wide, and having an open trussed roof. The light, placed about midway, lengthways, has a flat canvas frame, forming a sort of ceiling directly over it, to help to diffuse the illumination. The whole of the shed is well lit; and a large quantity of light also penetrates into an adjoining one of similar dimensions, and separated by a row of columns. The light is used regularly all through the night, and has been so all through the winter. Messrs. Tate speak highly of its efficiency. To ascertain the exact cost of the light, as well as of the gas illumination which it replaced, a gas-meter was placed to measure the consumption of the gas through the jets affected; and also the carbons consumed by the electric illumination were noted. A series of careful experiments showed that during a winter's night of 14 hours' duration the illumination by electricity cost 1s. 9d., while that by gas was 3s. 6d., or 11/2d. per hour against 3d. per hour. To this must be added the greatly increased illumination, four to five times, given by the electric light, to the benefit of the work; while this last illuminant also allowed, during the process of manufacture of the sugar, the delicate gradations of tint to be detected; and so to avoid those mistakes, sometimes costly ones, liable to arise through the yellow tinge of gas illumination. This alone would add much to the above-named economy, arising from the use of electric illumination in sugar works."

I am indebted for these facts to Mr. J. N. Shoolbred, under whose supervision the arrangements were made.

Some excellent experience has been gained at the shipbuilding docks in Barrow-in-Furness, where the Brush system has been applied to illuminate several large sheds covering the punching and shearing machinery, bending blocks, furnaces, and other branches of this gigantic business. In one shed, which was formerly lighted by large blast-lamps, in which torch oil was burnt, costing about 5d. per gallon, and involving an expenditure of L8 9s. per week, the electric light has been adopted at an expenditure of L4 14s. per week.

The erecting shop, 450 feet by 150 feet, formerly dimly lit by gas at a cost of L22 per week, is now efficiently lit by electricity at half the cost.

I am indebted for these facts to Mr. Humphreys, the manager of the works.

The Post office authorities have contracted with Mr. M. E. Crompton, to light up the Post-office at Glasgow for the same price as they have hitherto paid for gas, and there is no doubt that in many instances this arrangement will leave a handsome profit to the Electric Light Company. They are about to try the Brockie system in the telegraph galleries, and the Brush system in the newspaper sorting rooms of the General Post-office in St. Martin's-le-Grand.

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ON THE SPACE PROTECTED BY A LIGHTNING-CONDUCTOR.

By WILLIAM HENRY PREECE.

[Footnote: From the Philosophical Magazine for December, 1880.]

Any portion of non-conducting space disturbed by electricity is called an electric field. At every point of this field, if a small electrified body were placed there, there would be a certain resultant force experienced by it dependent upon the distribution of electricity producing the field. When we know the strength and direction of this resultant force, we know all the properties of the field, and we can express them numerically or delineate them graphically, Faraday (Exp. Res., Sec. 3122 et seq.) showed how the distribution of the forces in any electric field can be graphically depicted by drawing lines (which he called lines of force) whose direction at every point coincides with the direction of the resultant force at that point; and Clerk-Maxwell (Camb. Phil. Trans., 1857) showed how the magnitude of the forces can be indicated by the way in which the lines of force are drawn. The magnitude of the resultant force at any point of the field is a function of the potential at that point; and this potential is measured by the work done in producing the field. The potential at any point is, in fact, measured by the work done in moving a unit of electricity from the point to an infinite distance. Indeed the resultant force at any point is directly proportional to the rate of fall of potential per unit length along the line of force passing through that point. If there be no fall of potential there can be no resultant force; hence if we take any surface in the field such that the potential is the same at every point of the surface, we have what is called an equipotential surface. The difference of potential between any two points is called an electromotive force. The lines of force are necessarily perpendicular to the surface. When the lines of force and the equipotential surfaces are straight, parallel, and equidistant, we have a uniform field. The intensity of the field is shown by the number of lines passing through unit area, and the rate of variation of potential by the number of equipotential surfaces cutting unit length of each line of force. Hence the distances separating the equipotential surfaces are a measure of the electromotive force present. Thus an electric field can be mapped or plotted out so that its properties can be indicated graphically.



The air in an electric field is in a state of tension or strain; and this strain increases along the lines of force with the electromotive force producing it until a limit is reached, when a rent or split occurs in the air along the line of least resistance—which is disruptive discharge, or lightning.



Since the resistance which the air or any other dielectric opposes to this breaking strain is thus limited, there must be a certain rate of fall of potential per unit length which corresponds to this resistance. It follows, therefore, that the number of equipotential surfaces per unit length can represent this limit, or rather the stress which leads to disruptive discharge. Hence we can represent this limit by a length. We can produce disruptive discharge either by approaching the electrified surfaces producing the electric field near to each other, or by increasing the quantity of electricity present upon them; for in each case we should increase the electromotive force and close up, as it were, the equipotential surfaces beyond the limit of resistance. Of course this limit of resistance varies with every dielectric; but we are now dealing only with air at ordinary pressures. It appears from the experiments of Drs. Warren De La Rue and Hugo Muller that the electromotive force determining disruptive discharge in air is about 40,000 volts per centimeter, except for very thin layers of air.



If we take into consideration a flat portion of the earth's surface, A B (fig. 1), and assume a highly charged thunder-cloud, C D, floating at some finite distance above it, they would, together with the air, form an electrified system. There would be an electric field; and if we take a small portion of this system, it would be uniform. The lines, a b, a' b'...would be lines of force; and cd, c' d', c" d' ...would be equipotential planes. If the cloud gradually approached the earth's surface (Fig. 2), the field would become more intense, the equipotential surfaces would gradually close up, the tension of the air would increase until at last the limit of resistance of the air, e f, would be reached; disruptive discharge would take place, with its attendant thunder and lightning. We can let the line, e f, represent the limit of resistance of the air if the field be drawn to scale; and we can thus trace the conditions that determine disruptive discharge.



If the earth-surface be not flat, but have a hill or a building, as H or L, upon it, then the lines of force and the equipotential planes will be distorted, as shown in Fig. 3. If the hill or building be so high as to make the distance H h or L l equal to e f (Fig. 2), then we shall again have disruptive discharge.

If instead of a hill or building we erect a solid rod of metal, G H, then the field will be distorted as shown in Fig. 4. Now, it is quite evident that whatever be the relative distance of the cloud and earth, or whatever be the motion of the cloud, there must be a space, g g', along which the lines of force must be longer than a' a or H H'; and hence there must be a circle described around G as a center which is less subject to disruptive discharge than the space outside the circle; and hence this area may be said to be protected by the rod, G H. The same reasoning applies to each equipotential plane; and as each circle diminishes in radius as we ascend, it follows that the rod virtually protects a cone of space whose height is the rod, and whose base is the circle described by the radius, G a. It is important to find out what this radius is.



Let us assume that a thunder-cloud is approaching the rod, A B (Fig. 5), from above, and that it has reached a point, D', where the distance. D' B, is equal to the perpendicular height, D' C'. It is evident that, if the potential at D be increased until the striking-distance be attained, the line of discharge will be along D' C or D' B, and that the length, A C', is under protection. Now the nearer the point D' is to D the shorter will be the length A C' under protection; but the minimum length will be A C, since the cloud would never descend lower than the perpendicular distance D C.

Supposing, however, that the cloud had actually descended to D when the discharge took place. Then the latter would strike to the nearest point; and any point within the circumference of the portion of the circle, B C (whose radius is D B), would be at a less distance from D than either the point B or the point C.

Hence a lightning-rod protects a conic space whose height is the length of the rod, whose base is a circle having its radius equal to the height of the rod, and whose side is the quadrant of a circle whose radius is equal to the height of the rod.

I have carefully examined every record of accident that was available, and I have not yet found one case where damage was inflicted inside this cone when the building was properly protected. There are many cases where the pinnacles of the same turret of a church have been struck where one has had a rod attached to it; but it is clear that the other pinnacles were outside the cone; and therefore, for protection, each pinnacle should have had its own rod. It is evident also that every prominent point of a building should have its rod, and that the higher the rod the greater is the space protected.

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PHOTO-ELECTRICITY OF FLUOR-SPAR CRYSTALS.

Hantzel has communicated to the Saxon Royal Society of Science some interesting observations on the production of electricity by light in colored fluor-spar. The centers of the fluor-spar cubes become negatively electric by the action of light. The electric tension diminishes toward the edges and angles, and frequently positive polarity is produced there. With very sensitive crystals a short exposure to daylight is sufficient; by a long exposure to light the electric current increases. The direct rays of the sun act much more powerfully than diffused daylight, and the electric carbon light is more powerful even than sunlight. The photo-electric action of light belongs principally to the "chemically active" rays; this is shown by the fact that the production of electricity is extremely small behind a glass colored with cuprous oxide, and behind a film of a solution of quinine sulphate; while it is not appreciably diminished by a film of a solution of alum. The photo-electric excitability of fluor-spar crystals is increased by a moderate heat (80 deg. to 100 deg. C.).

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THE AURORA BOREALIS AND TELEGRAPH CABLES.

The January and February numbers of the Elektrotechnische Zeitschrift contain a number of articles on this interesting subject by several eminent electricians. Professor Foerster, director of the observatory in Berlin, points out the great importance of the careful study of earth currents, first observed at Greenwich, and now being investigated by a committee appointed by the German Government. He further points out, according to Professor Wykander, of Lund, in Sweden, that a close connection exists between earth currents, the protuberances of the sun, and the aurora borealis, and that the nearly regular periodical reappearance of protuberances in intervals of eleven years coincides with similar periods of excessive magnetic earth currents and the appearance of the aurora borealis. The remarkable disturbing influences on telegraph wires and cables of the aurora borealis observed from the 11th to 14th of August, 1880, have been carefully recorded by Herr Geh. Postnath Ludwig in Berlin, and a map of Europe compiled, showing the places affected, with the extent to which telegraph wires and cables were influenced and disturbed. Although the aurora was but faintly visible in England and Germany, and in Russia only as far as 35 deg. north, disturbing influences were reported from all parts of Europe, the Mediterranean, and Africa, and even Japan and the east coast of Asia. As far south as Zanzibar, Mozambique, and Natal disturbances were also noticed. They were in Europe most intense on the morning of August 12, when they lasted the whole day, and increased again in intensity toward eight o'clock in the evening, while they suddenly ceased everywhere almost simultaneously. Scientific and careful observations were only taken at a few places, but the existence of earth currents in frequently changing direction and varying intensity, was noticed everywhere. Long lines of wires were more affected than short ones, and although some lines—for instance the Berlin-Hamburg in an east-west direction—were not at all influenced, no general law was noticed according to which certain directions were freed from the disturbing influence. While, for instance, the Red Sea cable was not noticeably affected, the land line to Bombay, forming a continuation of this cable, was materially disturbed. The Marseilles-Algiers cable, so seriously influenced in 1871, showed no signs at all, but as may be expected, the north of Europe suffered more than the south, and in Nystad, Finland, the galvanometer indicated an intensity of current equal to that of 200 Leclanche cells.

Since thunderstorms are generally local, it is only natural that their effect upon telegraph cables should also be confined to one locality. Numerous careful observations, carried out over considerable periods of time, show that the disturbing influences of thunderstorms on telegraph lines are of less duration and more varying in direction and intensity than those of the aurora borealis. Long lines suffer less than short lines; telegraph wires above ground are more easily and more intensely affected than underground cables. It is, however, possible, that this is mainly due to the fact that in the districts where strict records were kept, in the German Empire, most of the long lines are underground cables, while most of the short local lines are overground wires. The results of the disturbances varied; in Hughes's apparatus the armatures were thrown off, lines in operation indicated wrong signs, dots became dashes, and the spaces were either multiplied in size or number, according to the direction of the earth currents induced by the thunderstorms. Since these observations extended over nearly 2,000 cases, some conclusions might fairly be drawn from them. For the purpose of a more complete knowledge on this subject, Dr. Wykander recommends a series of regular observations on earth currents to be carried out at different stations, well distributed over the whole surface of the globe, these observations to be made between six and eight A.M., and at the same time in the evening. Special arrangements to be made at various stations to record exceptionally intense disturbances during the phenomena of the aurora borealis, notice to be taken of time, direction, intensity, and all further particulars. Since this question appears to bear a considerable amount of influence on underground cables, it is one that deserves serious attention before earth cables are more generally introduced; there can, however, be little doubt that they are not nearly so much exposed as overhead wires to disturbing influences of other kinds, such as snow, rain, wind, etc., while they certainly do suffer, though perhaps in a less degree, by electrical disturbances.—Engineering.

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THE PHOTOGRAPHIC IMAGE: WHAT IT IS.

[Footnote: A communication to the Sheffield Photographic Society in the British Journal of Photography.]

It is quite possible that in the remarks I propose making this evening in connection with the photographic art I may mention topics and some details which are familiar to many present; but as chemistry and optical and physical phenomena enter largely into the theory and practice of photography, the field is so extensive there is always something interesting and suggestive even in the rudiments, especially to those who are commencing their studies. Although this paper may be considered an introductory one, I do not wish to load it with any historical account, or describe the early methods of producing a light picture, but shall at once take for my subject, "The Photographic Image: What It Is," and under this heading I must restrict myself to the collodion and silver or wet process, leaving gelatine dry plates, collodio-chloride, platinum, carbontype, and the numerous other types which are springing up in all directions for future consideration.

Now, in an ordinary pencil, pen and ink, or sepia sketch we have a deposit of a dark, non-reflecting substance, which gives the outline of a figure on a lighter background. The different gradations of shade are acquired by a more or less deposit of lead, ink, or sepia. In photography—at least in the ordinary silver process—the image is formed by a deposition of metallic silver or organic oxide in a minute state of division, either on glass, paper, or other suitable material. This is brought about by the action of light and certain reagents. Light has long been recognized as a motive power comparable with heat or electricity. Its action upon the skin, fading of colors, and effect on the growth of vegetable and animal organisms are well known; and, although the exact molecular change in many instances is not clearly understood, yet certain salts of silver, iron, the alkaline bichromates, and some organic materials—as bitumen and gelatine—have been pretty well worked out.

It is a remarkable and well-known fact that the chloride, iodide, and bromide of silver—called "sensitive salts" in photography—are not susceptible (at least only slowly) to change when exposed to the yellow, orange, and red rays. The longer wave lengths of the spectrum, as you know, form, with violet, indigo, blue, and green, white light. The diagram on the wall shows this dispersion and separation of the primitive colors. These—the yellow, orange, and red— are called technically "non actinic" rays, and the others in their order become more actinic until the ultra violet is reached. The action of white light, or rays, excluding yellow, orange, and red, has the effect of converting silver chloride into a sub-chloride; it drives off one equivalent of chlorine. Thus, silver chloride, Ag_2Cl_2=Ag_2Cl+Cl. When water is present the water is decomposed. Hydrochloric acid, HCl, hypochlorous acid, HClO is formed.

The iodide of silver in like manner is changed into a sub-iodide; but with water hydriodic acid is formed unless an iodine absorbent be present—then into hypoiodic acid. The silver bromide undergoes a similar change. When with light alone, a sub-bromide, Ag_2Br_2=Ag_2Br+Br, and with water hypobromous acid. It is important to bear this in mind, as one or other, and frequently both iodide and bromide of silver, is the sensitive salt requisite or used in producing the invisible image.

The theory regarding these sensitive salts of silver is that, being very unstable, i. e., ready to undergo a molecular change, the undulations produced in the ether, which pervades all space, and the potential action or moving power of light is sufficient to disturb their normal chemical composition; it liberates some of the chlorine, iodine, or bromine, as the case may be. This action, of course, applies to light from any source—the sun, electricity, or the brighter hydrocarbons, also flame from gas or candle, whether it comes direct as rays of white light or is reflected from an object and conducted through a lens as a distinct image upon the screen of a camera.

I have no time to speak on the subject of lenses, only just to mention that they are, or ought to be, achromatic, so as to transmit white light and of perfect definition, and the amount of light passed through should be as much as possible consistent with a sharp image—at least when rapid exposure is attempted.

I shall touch very lightly on the manipulative part of photography, as that would be unnecessary; but a brief account of the chemicals in use is essential to a right appreciation of the theory of developing the image. In the first place, our object is to get a film of some suitable material coated with a thin layer of a sensitive salt of silver—say a bromo-iodide. By mixing certain proportions of ammonium iodide and cadmium bromide, or an iodide and bromide of cadmium with collodion—which is pyroxyline, a kind of gun-cotton dissolved in ether and alcohol—a plate of glass is coated, and before being perfectly dry is immersed in the nitrate of silver bath. The silver nitrate solution, adhering and entering to a slight extent the surface of the collodion, becomes converted by an ordinary chemical action of affinity into silver iodide and bromide.

The ammonium and cadmium play a secondary part in the process, and are not absolutely necessary in forming the image. The plate is now extremely sensitive to light. When we have entered it into the dark slide and camera, and then exposed to light, the change I mentioned has taken place. The film is transformed into different quantities of sub-iodide and sub-bromide of silver, according to brilliancy of light. In addition, there is on the plate an amount of unchanged silver nitrate which becomes useful in the second stage, or development. The image is not seen as yet, being latent, and requiring the well-known developing solution of sulphate of iron, acetic acid, alcohol, and water. Practically we all recognize the effect of a nicely-balanced wave of developer worked round a plate. The high lights are first to appear as a darker color, till the details of shadow come out; when this is reached the developer is washed off. The chemical action is briefly thus, and it can be shown by solutions without a photographic plate, as in a test tube: Pour into this glass a solution of silver nitrate, AgNO, and add a solution of ferrous sulphate, FeSO_4. The ferrous sulphate combines with the nitric acid, forming two new salts—ferric nitrate and ferric sulphate. The silver is deposited. Any other substance which will remove oxygen from silver nitrate without combining with the silver would do the same, and metallic silver would be thrown down. The formula, as shown on the diagram, explains the interchange.

When the developer is poured over the plate it attacks first the free silver nitrate, and causes it to deposit extremely fine particles of metallic silver. The question arises: How is it these particles arrange themselves to form an image? This is explained by the physical movement known as molecular attraction or affinity. These particles are attracted first to the portions of the plate where there is most sub-iodide and sub-bromide. In the shady parts less silver is deposited. When the image is once started it follows that particles of silver produced by the iron developer will cause more to fall down on the face of those already present, and the image is, of course, built up if the silver nitrate be all consumed on the plate. The developer then becomes useless or injurious. The presence of acetic acid checks the reduction of the silver, and the alcohol facilitates the flow when the bath becomes charged with ether and spirit.

The molecular attraction just mentioned is made plainer by reference to the simple lead tree experiment. We have here in this bottle a piece of zinc rod introduced into a solution of acetate of lead. A chemical change has taken place. The zinc has abstracted the acetic acid and the lead is deposited on the zinc, and will continue to be so until the solution is exhausted. The irregularities of surface and arborescent appearance are well shown. If the change were rapidly conducted the lead particles would from their weight sink directly to the bottom instead of aggregating together like ordinary crystals. I have constructed a diagram of colored card, which will perhaps more clearly demonstrate the relation of the different constituents. The lower portion (Fig. a) represents a section of the glass plate or support, the collodion film (Fig. b) having upon its surface a thin layer of bromo-iodine silver (Fig. c), which, when exposed to a well-lighted image, as in a camera, changes into different gradations of sub-bromide and sub-iodide, as indicated by irregular, dark masses in the film. The dotted marks immediately above these are intended for the silver deposit (Fig. d)—clusters of granules, more abundant in the well lighted and less in the shaded parts of the picture, corresponding to the amount of sub-bromide and iodide beneath.



The next point to consider is that of intensification—a process seldom required in positive pictures, and would not be needed so often in negatives if there was enough free silver nitrate on the plate during development. The object, as we all know, in a wet-plate negative is to get good printing density without destruction of half-tone. It is a rule, I believe, in an over-exposed picture to intensify after fixing the image, and in an under-exposed picture to intensify before fixing. Whichever is done the intention is similar, namely, to intercept in a greater degree the light passing through a negative, so as to make a whiter and cleaner print. The usual intensifier—and, I suppose, there is no better—is pyrogallic acid, citric acid, water, and a few drops of silver nitrate solution. Pyrogallic is the most active agent, and might be used alone with water; but for special reasons it is not desirable. As a chemical it has a great affinity for oxygen, and will precipitate silver from a solution containing, for instance, nitrate of silver. It also combines with the metal, forming a pyrogallate—a dark brown, very non-actinic material. The use of a few drops of AgNO_3 solution is very evident. A deposit is added to the image already formed. Citric acid is the retarder in this case. Alcohol is unnecessary, as the film is well washed with water before the intensifier is used, consequently it flows readily over the plate.

As regards fixing, or, more properly, clearing the image: it is the simple act of dissolving out or from the film all free nitrate, chloride, iodide, or bromide. Cyanide of potassium does not attack the metallic deposit unless very strong. It has then a tendency to reduce the detail in the shadows.

THOMAS H. MORTON, M.D.

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GELATINE TRANSPARENCIES FOR THE LANTERN.

[Footnote: A communication to the Photographic Society of Ireland.]

Few of those who work with gelatine dry plates seem to be aware of the great beauty of the transparencies for lantern or other uses which can be made from them by ferrous oxalate development with the greatest ease and certainty.

I think this a very great pity, for I hold the opinion that the lantern furnishes the most enjoyable and, in some cases, the most perfect of all means of showing good photographic pictures. Many prints from excellent negatives which may be passed over in an album without provoking a remark will, if printed as transparencies and thrown on the screen, call forth expressions of the warmest admiration; and justly so, for no paper print can do that full justice to a really good negative which a transparency does. This difference is more conspicuous in these days of dry gelatine plates and handy photographic apparatus, when many of our most interesting negatives are taken on quarter or 5 x 4 plates the small size of which frequently involves a crowding of detail, much of which will be invisible in a paper print, but which, when unraveled or opened out, as it were, by means of the lantern, enhances the beauty of the pictures immensely.

When I last had the pleasure of bringing this subject before the members of our society, it may be remembered that I demonstrated the ease and simplicity with which those beautiful results maybe obtained, by printing in an ordinary printing frame by the light of my petroleum developing lamp, raising one of its panes of ruby glass for the purpose for five seconds, and then developing by ferrous oxalate until I got the amount of intensity requisite. On that evening, in the course of a very just criticism by one of our members, Mr. J. V. Robinson, he pointed out what was undoubtedly a defect, viz., a slightly opalescent veiling of the high lights, which should range from absolutely bare glass in the highest points. He showed that, in consequence of this veiling, the light was sensibly diminished all over the picture. This veiling of the high lights was a serious disadvantage in another important particular, inasmuch as it lessened the contrast between the lights and shadows of the picture, thereby robbing it of some of its charm and deteriorating its quality.

Since that evening I have endeavored, by a series of experiments, to find out some means by which this opalescence might be got rid of in the most convenient manner. Cementing the transparency to a piece of plain, clear glass with Canada balsam, as suggested by Mr. Woodworth, I found in practice to be open to two formidable objections. One of these was that Canada balsam used in this manner is a sticky, unpleasant substance to meddle with, and takes a long time—nearly a month—to harden when confined between plates in this manner. The other objection was of extreme importance, namely, that, in consequence of commercial gelatine plates not being prepared on perfectly flat glasses in all cases, I found that, after squeezing out the superfluous balsam and the air bubbles that might have formed from between the two plates, they are liable to separate at the places where the transparency is not flat, causing air bubbles to creep in from the edges, as you may see from these examples. I, therefore, have discarded this method, although it had the effect desired when successfully done.

I have hit, however, upon another way of utilizing Canada balsam, which, while retaining all the good qualities of the former method, is not subject to any of its disadvantages. This consists in diluting the balsam with an equal bulk of turpentine, and using it as a varnish, pouring it on like collodion, flowing it toward each corner, and pouring it off into the bottle from the last corner, avoiding crapy lines by slowly tilting the plate, as in varnishing. If the plate be warmed previously, the varnish flows more freely and leaves a thinner coating of balsam behind on the transparency. When the plate has ceased to drip, place it in a plate drainer, with the corner you poured from lowest, and leave it where dust cannot get at it for four or five days, when it will be found sufficiently hard to be put into a plate box. The transparency may be finished at any time afterward by putting a clean glass of the same size along with it, placing one of the blank paper masks sold for the purpose—either circular or cushion-shaped to suit the subject—between the plates, and pasting narrow strips of thin black paper over the edges to bind them together. This method is very successful, as you may see from the examples. It renders the high lights perfectly clear, and leaves a film like glass over all the parts of the transparency where the varnish has flowed.

In order to avoid the risk of dust involved in this process, I tried other means of arriving at similar results and with success, for the plates I now submit to you have been simply rubbed or polished, as I may say, with a mixture of one part of Canada balsam to three parts of turpentine, using either a small tuft of French wadding or a small piece of soft rag for the purpose, continuing the rubbing until the plate is polished nearly dry. This method is particularly successful, rendering the clear parts of the sky like bare glass. I have here a plate which is heavily veiled—almost fogged, in fact—one half of which I have treated in this way, showing that the half so treated is beautifully clear, while the other half is so veiled as to be apparently useless.

I have tried to still further simplify this necessary clearing of those plates, and find that soaking tor twelve hours in a saturated solution of alum, after washing the hypo out of the plate, is successful in a large number of cases; and where it is successful there is no further trouble with the transparency, except to mount it after it becomes dry. Where it is not entirely successful I put the plate into a solution of citric acid, four ounces to a pint of water, for about one minute, and have in nearly all cases succeeded in getting a beautifully-clear plate. The picture must not be left long in the citric acid solution, or it will float off; neither do I like using citric acid until after trying the alum, for a similar reason.

I may mention that I recommend a short exposure in the printing-frame and slow development, in order to get sufficient intensity. Of course the exposure is always made to a gas or petroleum light. I also still prefer the old method of making the ferrous oxalate solution, pouring it back into the bottle each time after using, and using it for two or three months, keeping the bottle full from a stock bottle, and occasionally putting a little dry ferrous oxalate into the bottle and shaking it up, allowing it to settle before using next time. By treating it in this way it retains its power fairly well for a long time; and as it becomes less active I give a little longer exposure, balancing one against the other. Making the ferrous oxalate solution from two saturated solutions of iron sulphate and potassium oxalate has not succeeded so well with me for transparencies. The tone of the picture is not so black as when developed by the old method; and I do not like gray transparencies for the lantern. I also recommend very slow gelatine plates, about twice as sensitive as wet collodion—not more, if I can help it.

I have demonstrated, I hope to your satisfaction, the possibility of producing lantern slides from commercial gelatine plates of a most beautiful quality—ranging from clear glass to deep black, and giving charming gradation of tones, showing on the screen a film as structureless as albumen slides, without the great trouble involved in making them. You must not accept the slides put before you this evening as the best that can be done with gelatine. Far from it; they are only the work of an amateur with very little leisure now to devote to their manufacture, and are merely the result of a series of experiments which, so far as they have gone, I now place before you.—Thomas Mayne, T. C., in British Journal of Photography.

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AN INTEGRATING MACHINE.

[Footnote: Read at a meeting of the Physical Society, Feb. 26.]

By C.V. BOYS.

All the integrating machines hitherto made, of which I can find any record, may be classed under two heads, one of which, Ainslee's machine, is the sole representative, depending on the revolution of a disk which partly rolls and partly slides on the paper, and the other comprising all the remaining machines depending on the varying diameters of the parts of a rolling system. Now, none of these machines do their work by the method of the mathematician, but in their own way. My machine, however, is an exact mechanical translation of the mathematical method of integrating y dx, and thus forms a third type of instrument.

The mathematical rule may be described in words as follows: Required the area between a curve, the axis of x and two ordinates; it is necessary to draw a new curve, such that its steepness, as measured by the tangent of the inclination, may be proportional to the ordinate of the given curve for the same value of x, then the ascent made by the new curve in passing from one ordinate to the other is a measure of the area required.

The figure shows a plan and side elevation of a model of the instrument, made merely to test the idea, and the arrangement of the details is not altogether convenient. The frame-work is a kind of T square, carrying a fixed center, B, which moves along the axis of x of the given curve, a rod passing always through B carries a pointer, A, which is constrained to move in the vertical line, ee, of the T square, A then may be made to follow any given curve. The distance of B from the edge, ee, is constant; call it K, therefore, the inclination of the rod, AB, is such that its tangent is equal to the ordinate of the given curve divided by K; that is, the tangent of the inclination is proportional to the ordinate; therefore, as the instrument is moved over the paper, AB has always the inclination of the desired curve.

The part of the instrument that draws the curve is a three-wheeled cart of lead, whose front wheel, F, is mounted, not as a caster, but like the steering wheel of a bicycle. When such a cart is moved, the front wheel, F, can only move in the direction of its own plane, whatever be the position of the cart; if, therefore, the cart is so moved that F is in the line, ee, and at the same time has its plane parallel to the rod, AB, then F must necessarily describe the required curve, and if it is made to pass over a sheet of black tracing paper, the required curve will be drawn. The upper end of the T square is raised above the paper, and forms a bridge, under which the cart travels. There is a longitudinal slot in this bridge in which lies a horizontal wheel, carried by that part of the cart corresponding to the head of a bicycle. By this means the horizontal motion communicated to the front wheel of the cart by the bridge, is equal to that of the pointer, A; at the same time the cart is free to move vertically.

The mechanism employed to keep the plane of the front wheel of the cart parallel to AB is made clear by the figure. Three equal wheels at the ends of two jointed arms are connected by an open band, as shown. Now, in an arrangement of this kind, however the arms or the wheels are turned, lines on the wheels, if ever parallel, will always be so. If, therefore, the wheel at one end is so supported that its rotation is equal to that of AB, while the wheel at the other end is carried by the fork which supports F, then the plane of F, if ever parallel to AB, will always be so. Therefore, when A is made to trace any given curve, F will draw a curve whose ascent is (1/K) f y dx, and this, multiplied by K, is the area required.



Not only does the machine integrate y dx, but if the plane of the front wheel of the cart is set at right angles instead of parallel to AB, then the cart finds the integral of dx / y, and thus solves problems, such, for instance, as the time occupied by a body in moving along a path when the law of the velocity is known.

Some modifications of the machine already described will enable it to integrate squares, cubes, or products of functions, or the reciprocals of any of these.

Of the various curves exhibited which have been drawn by the machine, the following are of special physical interest.

Given the inclined straight line y = cx, the machine draws the parabola y = cx squared / 2. This is the path of a projectile, as the space fallen is as the area of the triangle between the inclined line, the axis of x, and the traveling ordinate.

Given the curve representing attraction y = 1 / x squared the machine draws the hyperbola y = 1 / x the curve representing potential, as the work done in bringing a unit from an infinite distance to a point is measured by the area between the curve of attraction, the axis of x, and the ordinate at that point.

Given the logarithmic curve y = e^x, the machine draws an identical curve. The vertical distance between these two curves, therefore, is constant; if, then, the head of the cart and the pointer, A, are connected by a link, this is the only curve they can draw. This motion is very interesting, for the cart pulls the pointer and the pointer directs the cart, and between they calculate a table of Naperian logarithms.

Given a wave-line, the machine draws another wave-line a quarter of a wave-length behind the first in point of time. If the first line represents the varying strengths of an induced electrical current, the second shows the nature of the primary that would produce such a current.

Given any closed curve, the machine will find its area. It thus answers the same purpose as Ainslee's polar planimeter, and though not so handy, is free from the defect due to the sliding of the integrating wheel on the paper.

The rules connected with maxima and minima and points of inflexion are illustrated by the machine, for the cart cannot be made to describe a maximum or a minimum unless the pointer, A, crosses the axis of x, or a point of inflexion unless A passes a maximum or minimum.

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UPON A MODIFICATION OF WHEATSTONE'S MICROPHONE AND ITS APPLICABILITY TO RADIOPHONIC RESEARCHES.

[Footnote: A paper read before the Philosophical Society of Washington. D. C., June 11, 1881.]

By ALEXANDER GRAHAM BELL.

In August, 1880, I directed attention to the fact that thin disks or diaphragms of various materials become sonorous when exposed to the action of an intermittent beam of sunlight, and I stated my belief that the sounds were due to molecular disturbances produced in the substance composing the diaphragm.[1] Shortly afterwards Lord Raleigh undertook a mathematical investigation of the subject and came to the conclusion that the audible effects were caused by the bending of the plates under unequal heating.[2] This explanation has recently been called in question by Mr. Preece,[3] who has expressed the opinion that although vibrations may be produced in the disks by the action of the intermittent beam, such vibrations are not the cause of the sonorous effects observed. According to him the aerial disturbances that produce the sound arise spontaneously in the air itself by sudden expansion due to heat communicated from the diaphragm—every increase of heat giving rise to a fresh pulse of air. Mr. Preece was led to discard the theoretical explanation of Lord Raleigh on account of the failure of experiments undertaken to test the theory.

[Footnote 1: Amer. Asso. for Advancement of Science, August 27, 1880.]

[Footnote 2: Nature, vol. xxiii., p. 274.]

[Footnote 3: Roy. Soc., Mar. 10, 1881.]



He was thus forced, by the supposed insufficiency of the explanation, to seek in some other direction the cause of the phenomenon observed, and as a consequence he adopted the ingenious hypothesis alluded to above. But the experiments which had proved unsuccessful in the hands of Mr. Preece were perfectly successful when repeated in America under better conditions of experiment, and the supposed necessity for another hypothesis at once vanished. I have shown in a recent paper read before the National Academy of Science,[1] that audible sounds result from the expansion and contraction of the material exposed to the beam, and that a real to-and-fro vibration of the diaphragm occurs capable of producing sonorous effects. It has occurred to me that Mr. Preece's failure to detect, with a delicate microphone, the sonorous vibrations that were so easily observed in our experiments, might be explained upon the supposition that he had employed the ordinary form of Hughes's microphone shown in Fig. 1, and that the vibrating area was confined to the central portion of the disk. Under such circumstances it might easily happen that both the supports (a b) of the microphone might touch portions of the diaphragm which were practically at rest. It would of course be interesting to ascertain whether any such localization of the vibration as that supposed really occurred, and I have great pleasure in showing to you tonight the apparatus by means of which this point has been investigated (see Fig. 2).

[Footnote 1: April 21, 1881.]



The instrument is a modification of the form of microphone devised in 1872 by the late Sir Charles Wheatstone, and it consists essentially of a stiff wire, A, one end of which is rigidly attached to the center of a metallic diaphragm, B. In Wheatstone's original arrangement the diaphragm was placed directly against the ear, and the free extremity of the wire was rested against some sounding body—like a watch. In the present arrangement the diaphragm is clamped at the circumference like a telephone diaphragm, and the sounds are conveyed to the ear through a rubber hearing tube, c. The wire passes through the perforated handle, D, and is exposed only at the extremity. When the point, A, was rested against the center of a diaphragm upon which was focused an intermittent beam of sunlight, a clear musical tone was perceived by applying the ear to the hearing tube, c. The surface of the diaphragm was then explored with the point of the microphone, and sounds were obtained in all parts of the illuminated area and in the corresponding area on the other side of the diaphragm. Outside of this area on both sides of the diaphragm the sounds became weaker and weaker, until, at a certain distance from the center, they could no longer be perceived.

At the point where we would naturally place the supports of a Hughes microphone (see Fig. 1) no sound was observed. We were also unable to detect any audible effects when thepoint of the microphone was rested against the support to which the diaphragm was attached. The negative results obtained in Europe by Mr. Preece may, therefore, be reconciled with the positive results obtained in America by Mr. Tainter and myself. A still more curious demonstration of localization of vibration occurred in the case of a large metallic mass. An intermittent beam of sunlight was focused upon a brass weight (1 kilogramme), and the surface of the weight was then explored with the microphone shown in Fig. 2. A feeble but distinct sound was heard upon touching the surface within the illuminated area and for a short distance outside, but not in other parts.

In this experiment, as in the case of the thin diaphragm, absolute contact between the point of the microphone and the surface explored was necessary in order to obtain audible effects. Now I do not mean to deny that sound waves may be originated in the manner suggested by Mr. Preece, but I think that our experiments have demonstrated that the kind of action described by Lord Raleigh actually occurs, and that it is sufficient to account for the audible effects observed.

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