A Same-size Pantograph.—For making a same-size copy, tracing may be preferred to the use of a pantograph; but if a pantograph is adopted, a special apparatus may be constructed for the purpose. The arrangement is exactly the same as that already described, excepting that the only holes needed are those at a, c, d, f, at the middle points of the four rods, the parallelogram formed by the rods being equal-sided. The fixed pivot is situated at d, and pencil and pointer holes are made at a and f.

Using the Pantograph.—When adjusting the instrument for reduction or enlargement, make sure that the adjustment pivots are in the holes corresponding with the scale. The fixed pivot, pointer, and pencil must be rigid, and, with pivot c, be of such a length that the pantograph as a whole moves parallel to the paper. A little sliding weight to place on the rod near the pencil will be found useful for keeping the pencil point in constant contact with the paper.

If the apparatus works stiffly, ease the holes a trifle and lead-pencil the wood at all points where two surfaces rub. It is absolutely impossible to make a good reproduction with a stiff, jerky pantograph.

To decide the positions of original and the paper for the copy, get the pointer centred on the original and adjust the paper till its centre is under the pencil.



XXXVII. A SILHOUETTE DRAWING MACHINE.

With this very simple apparatus you will be able to give good entertainment to such of your friends as may wish to have black paper records of their faces in profile.

The machine is merely a long rod, with a sliding pencil attached to one end and a metal pointer stuck into the other, supported near the pencil end on a pivot which permits free movement in all directions.

For heads and busts only, the rod and pointer combined need not be more than 4 feet 6 inches long. The rod is a 1/2-inch blind rod, the pointer a stout knitting-needle driven axially into one end of the rod. This pointer, being of small diameter, follows the minor curves and angles of the features much more closely than would be possible with the rod.

The support is a piece of wood, 1-1/2 inches square and 12 to 15 inches long, screwed on to a large foot, which should be fairly heavy, as any tilting or slipping will, of course, spoil the silhouette. The universal joint for the rod is made by soldering a small U-shaped piece of metal to the end of a short metal bar. The ends of the U are drilled for a pin passing through the rod; and a hole is sunk into the top of the support to take the bar. The fit should be close, to prevent the pivot rocking about, and the hole in the support deep enough to bring the bottom of the stirrup down against the wood.

If a series of holes half an inch apart is drilled, through the rod, the nearest 9 inches from the pencil end, the size of the silhouette proportionately to the original can be varied by moving the pin from one hole to another.



The pencil holder is 4 inches of tubing, in which the pencil can slide easily without shaking. If necessary, the size of the pencil should be reduced by rubbing with glass paper. Bind the holder tightly to the end of the rod away from the pointer, so that one extremity just overhangs the rod. A piece of thin elastic is tied to the unsharpened end of the pencil and to the pencil tube, the adjustment allowing the pencil to project an inch when the elastic is taut but not stretched.

A fairly soft pencil and a thick, smooth paper or card give the best results. Paper should be backed by something hard to prevent the pencil digging in. Attach the paper to a firm vertical surface, such as the side of a box, a drawing board, a wall, etc.

Using the Machine.—The rod support, paper, and sitter should be arranged so that the rod is level at the height of the sitter's nose and the pencil on the centre of the paper. Bring the support near enough to the paper to drive the pencil back into the tube until the point projects only half an inch.

A thread attached to the pencil will enable you to keep the pencil off the paper until you wish to begin drawing the profile.

Begin with the pointer pressing against the sitter's chest, and bring it over the face and down the back of the head and neck. Do not press it into the hair, but carry it along what you consider to be the outline; though it must be in actual contact with the features and clothes. It is hardly necessary to mention that the sitter must keep perfectly still if the silhouette is to be at all accurate.

The tracing is cut round with fine-pointed scissors, and the paper blacked and stuck on a piece of white card. Some trouble is saved by using paper white on one aide and black on the other. If duplicates are needed, two or more pieces of paper should be stuck together by the corners and to the paper on which the silhouette is drawn, and all be cut through at one operation.

With a little practice the actual tracing of the outline occupies but a few seconds. Things are expedited if an assistant adjusts the paper and pencil.



XXXVIII. A SIGNALLING LAMP.

Visual signalling is effected at night in the Morse code by means of a lamp fitted with an easily-moved shutter, which passes or cuts off the light at the will of the operator. Readers who know the Morse code might well go to the trouble of constructing in duplicate the simple apparatus to be described, as the possession of an outfit will enable them to extend their signalling capabilities.

The stand for the lamp is admirably supplied by the ordinary camera tripod. For the illuminant we may select any good acetylene cycle lamp.

For this a holder is made of 1/2-inch wood, according to the sketch shown in Fig. 189. The width of all the four parts should be about 2 inches greater than the front glass of the lamp. B and C should be sufficiently far apart to allow the lamp to rest on the rim above the carbide chamber; and the front, A, should be at least an inch higher than the top of the lamp glass.



The hole cut in B must be so situated as to bring the front of the lamp close to the front of the holder, so that the greatest possible amount of light may be utilized. The hole in A should be rather larger than the lamp front, and, of course, be accurately centred. Mark these two holes off carefully, and cut out with a pad saw or fret saw.

A socket must be attached to the centre of the underside of the base to take the camera screw; or, if such a socket is not easily obtainable, a hole should be drilled in the base to take an ordinary wood screw of good size, the surplus of which is cut off so as not to interfere with the lamp.

The Shutter.—The woodwork is so simple that nothing further need be said about it. The more difficult part of the business is the making of the shutter, which must be so constructed that it can be opened and closed rapidly by motions similar to those used in working the telegraph key described in a preceding chapter. Speed of working is obtained by dividing the shutter into two or three parts, each revolving on its own spindle, but all connected so as to act in perfect unison. The thinnest sheet brass or iron obtainable should be used, so that the tension of the spring used to close the shutter need not be great. Our illustration shows a two-part shutter, each half an inch wider than the hole in the front, and jointly a similar amount deeper. The upper half overlaps the lower, outside, by a quarter of an inch.

The spindles are two straight pieces of brass wire, revolving in sockets which are most easily made of notched pieces of wood (as shown in Fig. 189), with removable caps of strip tin. The lower spindle should be an inch longer than the width of the front, to allow for a cranked end, to which the closing spring will be attached.

Having cut out the halves of the shutter, solder the spindle wires to one edge of each on what will be the back side. The wires must be so arranged as to allow a quarter of an inch to project beyond the left edge of the front, as the opening mechanism is situated on this side as the most convenient for the operator.

Take a couple of metal discs, an inch or so in diameter, and bore a hole in each near the circumference to fit the ends of the pivots fairly tight. Three-eighths of an inch from this—centre to centre—bore and tap a hole for a small screw. The tapping should be done with a taper tap and carried just so far that the screw turns stiffly without danger of being broken off by the screw-driver.

Next find the correct positions of the parts of the shutter and the spindle sockets on the front of the holder, and mark them off carefully. Screw the wooden parts of the sockets to the front. Four little "distance pieces" should now be cut out of small tubing, or made by twisting tin round the spindle, to place on the spindles between shutter and sockets, so that the shutters cannot shift sideways.

The right-hand end of the lower spindle must be bent over (after slipping on the distance piece) to form a 1/2-inch crank making an angle of 45 degrees with the line of the front, in an upward direction, as it will be depressed by the opening of the shutter. Flatten out the end with a hammer, and drill a small hole near the tip.

The shutters can now be placed in position, and the caps of the sockets be screwed on. The next thing to make is the connecting rod to join the cranks at the left side of the front. For this purpose we may use a piece of fairly stiff strip metal—brass by preference—5 or 6 inches long. Half an inch from one end make a mark with the centre punch; then measure off exactly the distance between the shutter spindles, and make a second punch mark. Drill holes at the marks large enough, for the disc screws to pass through easily, but not loosely.

Attach the rod to the discs by the screws, and slip the discs on to the ends of the shutter spindles. (The free end of the rod should be upwards.) Press the shutters against the front so that they cannot open, adjust the discs at an angle of 45 degrees to the front in an upward direction, and solder them firmly to the spindles.

The upper end of the connecting rod should be turned over to form a finger rest, or be sharpened off to take a knob. The last operation is the fitting of the spring to close the shutter. A spiral spring attached at one end of the crank on the lower spindle and at the other to a nail projecting from the side of the front is the most convenient arrangement. If you have not got a spiral spring, you can easily make a. fairly efficient substitute out of hard brass wire wound a few times round a large wire nail.

An alternative method of springing is to add an arm, a, to the connecting rod, as shown by dotted lines in Fig. 189, and to use the projection for engaging a spring, made by winding hard brass wire a few times round a nail. A screw passed through the coil holds it to the front.

The tension of the spring must be just sufficient to close the shutter smartly and prevent it rebounding far enough to pass any light.



XXXIX. A MINIATURE GASWORKS.

The most primitive method of making coal gas on a small scale is to fill a tin—which must have folded, not soldered, joints—with small coal, punch a hole in the bottom, and place it lid downwards in the fire. Gas soon begins to issue, but, owing to the quantity of moisture and impurities present, it will not ignite until some minutes have elapsed. The flame, when it does make its appearance, is very smoky and gives little light, because, in addition to the coal gas of commerce, there are present ammonia gas, sulphuretted hydrogen, carbonic acid, tar vapour, etc., which prevent brightness of flame.



A miniature gasworks, if it is to be worthy of its name, must obviously endeavour to separate the troublesome components from the useful gas. The doing of this involves several processes, all simple enough in principle, and requiring but simple apparatus for demonstration on a small scale. To take them in order the processes are—

(l) The formation of gas in a retort;

(2) The condensation of the tar;

(3) The condensation of steam;

(4) The removal of the ammonia gas;

(5) The removal of the sulphuretted hydrogen and carbonic acid.

The last two processes are, in a real gasworks, usually separated, but for simplicity's sake we will combine them. Finally, the storage of the gas has to be provided for.

The Retort.—To get very good results, the retort should be of cast iron, and have a removable air-tight cover; but, to keep down expense, we will use an ordinary 2-pound self-opening coffee tin. A short piece of brass pipe is soldered into the lid near one edge to carry off the gas as it is generated. To get a fairly gas-tight joint, red-leaded asbestos string should be rammed tightly between the lid and the tin. The tin may be laid on an open fire on the slant, the lid end uppermost, and the pipe at the top, where the gas will collect; or, if you wish to make things more realistic, you may easily construct an oven with sides and back of fire-brick, and front of sheet iron, through the hole in which the tin is pushed horizontally, so that only half an inch projects. This is a. suitable arrangement for out of doors.



The Hydraulic Main.—This is represented in Fig. 190 by a double-necked bottle, B, standing in a bowl of cold water. The pipe from the retort passes through the cork in one neck and dips half an inch below the surface of the water inside. The gas, on meeting the water, is cooled, and some of the steam in it is condensed, also most of the tar present, which floats on the top of the water. From the bottle the gas passes on to the Condensers, where the process of cooling is completed gradually. The condenser (Fig. 191) is so designed as to cause the gas to pass through several pipes in succession. The base consists of a tin box, 6 inches long, 4 wide, and 1-3/4 deep. This is divided longitudinally down the centre by a 1-1/2-inch partition, soldered to the bottom and sides; and the two divisions are again subdivided, as shown in Fig. 192, by shorter cross partitions.



For the condensing pipes, "compo" tubing of 1/2-inch outside diameter is convenient. The amount required will, of course, depend on the number of pipes used and the length of the individual pipes. The design shows 6 pipes, each 3 feet long, bent to a semicircular curve (Fig. 191) at the middle to form very long, narrow horse-shoes. The pipes are supported at the curve by the crossbar, S (Fig. 191), of a frame, and their ends enter short pieces of brass tubing soldered into holes in the bottom of the tin box. Rubber bands make the joints air-tight.



The base is stood bottom upwards in a larger tin containing an inch and a half of water. The water acts as a seal, preventing the passage of the gas from one compartment to another through the pipes which it traverses, in the order indicated by the arrows and numbers in Fig. 192, to reach the outlet. On its way the gas is deprived of any water and of any traces of tar. The condensed water and tar fall from the open ends of the pipes into the base.

The Purifier is made of a large tin with overlapping lid. Near the bottom is soldered on an inlet pipe; just below the lid an outlet pipe. Cut out two discs of perforated zinc or sheet tin to fit inside the tin easily, but not loosely. (If tin is used, make a number of small holes in it.) The lower of the discs (Fig. 193, Bl) has three wire legs, AA, soldered to it, to support the upper disc, B. Three short supports keep it clear of the bottom.

The tin must be charged with a mixture of two parts green sulphate of iron and one part lime. The lime should be slaked a short time before use. The sulphate, lime, and sufficient water to moisten the whole are ground into a pulp and left to dry. The dry mixture, which has a reddish-yellow colour, is broken up fine. Put tray B1 into place and spread half the chemical over it; then lay B on the top and cover it with the remainder. The lid joint is sealed by a broad rubber band.

While passing through the tin, the ammonia, sulphuretted hydrogen and carbonic acid gases all combine with the chemical, and fairly pure gas issues from the outlet.

The Gasholder.—As the gasometer is an important feature of a gasworks, our small plant should contain its counterpart, as it serves to regulate the pressure of the gas, and, therefore, the steadiness of the flame, as well as affording storage room.

As a gasometer, one may use a container made on the principle of the lung-testing apparatus described on p. 361; or the gasholder of a lantern acetylene apparatus, which must, of course, be suitably counterweighted.

Working the Plant.—When starting up the plant, leave the burner open until inflammable gas issues, so that the air present in the various chambers may be displaced.

[Transcribers note: Premature lighting of the burner may cause the flame to propagate into the system and explode. I speak from experience.]

INDEX.

Aeroplane, model, self-launching. Bedplate for engine. Bellows, double. Bench, joiner's. Benham's harmonograph. Bicycle shed. Boilers, model. Bookstand. Box kites.

Cabinets, cardboard, cigar-box, match-box, tool. Circles, rolling. Clock, electric alarm. Colour top. Cylinder, double-acting steam.

Developing sink. Doors for shed. Double-acting horizontal steam engine. Double bellows.

Eccentrics. Electric alarm clock. Electric motor, reciprocating. Electric railway. Engine, hot-air. Experiments, apparatus for simple scientific.

Fuels for model boilers.

Gasworks, miniature. Ganges, rain, water, Gimbals, or universal joints. Gliders, paper. Goold's harmonograph. Governor for engine.

Harmonographs. Hot-air engines. House ladder.

Joiner's bench.

Kettles, quick-boiling. Kites, box. Kite winders.

Ladder, house. Lamp, signalling. Locomotive, electric. Lung-testing apparatus. Magic swingers. windmill. Match-boarding. Match-box, self-supplying. Morse code. Morse sounder. Motor, electric. Motor, water.

Nozzle for steam turbine.

Pantograph. Pendulums for harmonograph. Pens for harmonograph. Pneumatic puzzle. Poultry house. Propellers for aeroplane. Pumps. Puppets, wrestling. Puzzle, pneumatic.

Railway, electric. Rain gauges. Reciprocating steam engine, simple. Resistance, adjustable, for electric railway. Reversing switch for electric railway. Riveting.

Safety Valves. Sawing trestle. Shed for bicycle. Signalling lamp. Silhouette drawing machine. Simple scientific experiments. Sink, developing. Slide valve. Smoke-ring apparatus. Soldering. Spokes, magic. Steam cocks. Steam engines. Steam gauge. Steam pump. Steam tops. Steam turbines. Strength. testing machines. Swingers, magic. Switch, multiple battery. Switch, reversing.

Target apparatus. Telegraphic apparatus. Testing boilers. Tool cabinet. Top, colour. Tops, steam. Track for model railway. Trestle, sawing. Turbines, model steam.

Vanishing spiral. Vice for Joiner's bench.

Water gauge. Water motor. Weights for harmonograph pendulums. Windmill, magic. Wind vanes; electric. Workbox, Norwegian. Wrestling puppets. Wriggling line.

THE END.

PRINTED IN GREAT BRITAIN AT THE PRESS OF THE PUBLISHERS.