Fig. 503
A Hygroscope. The cottage is made of thin wood about 1/8 inch to 3/8 inch in thickness, according to measurements given in Fig. 501. The sides are about 3½ inches. The platform or floor on which it stands, 6½ inches by 4 inches. The sides of the roof are 4 inches by 4 inches, so that it projects slightly.
The doors in front are 1¼ inches wide and 3 inches high, and are cut out with the fret-saw; about half-an-inch of the partition between the doors is cut away to allow the disc on which the figures stand to swing round. Cut with fret-saw a circular disc of wood 1/8 inch thick, diameter 3 inches. Drill a hole through the centre and fit into it quite tightly a piece of wire bent into a loop as shown in Fig. 502. Drill a hole in the floor of the cottage, about an inch from the middle of the partition. The wire axle should fit into this so that it turns easily, but not too loosely, otherwise the disc on which the figures stand will wobble. Just over this hole there must be another hole in the roof. This can be made by filing, with round fret-saw file, a little hollow (Fig. 503) in each of the top sides of the roof, so that when they come together a hole is formed. The back, sides, floor and roof may now be nailed and glued together. Next cut out from three-ply wood with the fret-saw two little figures as in Fig. 501; they should be about 2 inches to 2¼ inches, and should be suitably coloured. These are glued to the wooden disc.
The disc is hung from the roof by a piece of catgut; a knot is made at the end to prevent it slipping through, the other end being tied to the wire loop; the wire passes through the hole in the floor. The catgut must be long enough to allow the disc to turn round completely on its axis. Four pieces of wood or four small reels are glued to the corners of the floor to prevent the wire axle from touching the ground. The front must not be put on until the model is found to work correctly. To do this, hang the disc so that it is parallel to the ground, and so that both figures are looking out of their respective doors; then tie the knot at the top and wait for a change of weather.
Supposing on a damp day the cricketer comes forward and the boy in mackintosh and sou'wester retires indoors, this is because the catgut is twisting the wrong way, therefore the end that is fastened to the roof must be fastened to the wire loop, and vice versa. Now the front can be glued on. It can be suitably painted, showing door-posts, windows, bricks, etc.
Why the Hygroscope works. Catgut has the peculiar property of absorbing moisture from the air and twisting up and becoming shorter; when the air is dry it untwists to its original length; the damper the air the greater is the amount of the twist. Hence in the model, as the catgut twists and untwists according to the state of the atmosphere, the little figures swing in and out of the cottage doors.
[CHAPTER XXV]
LIFT, PONT ROULANT, TOWER BRIDGE
