Now rule a line 5¾ inches from end a b. On this line screw in two small screw-eyes, K and L, of the same size as screw-eyes G and H. The axle, M N, may be either iron wire (in which case the bridge may work rather loosely) or, what is better, a wooden rod that just fits the screw-eyes. Whichever axle is chosen cork discs should be placed at each end to prevent it slipping out. Before the bridge is fastened on, screw-eyes O and P are screwed in it near the end c d. Screw-eye P must be far enough from the edge b d to clear screw-eye R when the bridge is upright. The same with screw-eye O.
A piece of strong thread is tied to screw-eye P, passed through screw-eye R, and through a hole in the drawbridge above screw-eye R, but clear of axle, M N. A similar piece of thread is tied to screw-eye O, passed through Q, and through a hole in the bridge.
Now cover up top, A B C D, with a piece of cardboard, but do not bring this quite up to B C, in order not to interfere with the working of the bridge. Make holes in the cardboard for the strings to pass through. Then cover up the front portion, D C F E, below the bridge with cardboard.
The tower (Fig. 512) must next be made. This is formed of one piece of cardboard: height, a b, 9 inches; width, c d, 3¼ inches.
In the sides facing the bridge large openings, E, are cut about 2½ inches high.
Small openings, F and G, about 1½ inches high and ¾ inch broad, are cut for the overhead foot bridges. These are made of long pieces of cardboard 2 inches broad, bent in three divisions to form the path and sides. The latter are marked to represent railings. They should be long enough to pass well inside the tower through openings F and G, and through the corresponding openings in the opposite tower. They can be glued into position by pieces of stripwood or left movable.
A door, A, should be made in the top of the tower and a platform put in to make a compartment for working the bridge. The pieces of thread are brought up through holes in this platform and fastened to rod B, which passes through holes in sides of tower, and is kept from slipping out by cork discs. When this rod is turned the bridge will rise or fall.
If a large model is being made a proper little windlass with a handle can be constructed inside the upper room of the tower. The threads pass up on each side of the tower so as not to interfere with the "traffic" passing under the arch of the bridge. The tower is fastened up with flanges and glued to the wooden box with the help of small blocks of wood. A square pyramid is placed on the top of the tower, and the whole is suitably coloured. A picture of the real Tower Bridge is a great help when finishing off the model.
A similar bridge and tower are made for the other side.