The combined axle and hook should be made as follows:—­Take a piece of thin steel wire, sharpen one end, and bend it as shown at C (Fig. 136).  Pass the end B through a tight-fitting hole in the centre of the small boss of the propeller, and drive C into the wood.  Solder a tiny piece of 1/8-inch brass tubing to the wire axle at A, close up to the rubber hook side of the propeller, and file quite smooth.  The only things now left to do are to bend the wire into the form of a hook (as shown by the dotted line), and to cover this hook, as already advised, with a piece of valve tubing to prevent fraying the rubber skeins.

[Illustration:  Fig. 136.—­Axle and hook for propeller.]

Weight.—­The weight of a model with a T-shaped central rod 1/16 inch thick should be 4-1/2 oz.  Probably it will be more than this—­as a maximum let us fix 6 oz.—­although 4-1/2 oz. is quite possible, as the writer has proved in actual practice.  In any case the centre of gravity of the machine without the rubber motor should be situated 1 inch behind the front or entering edge of the main plane.  When the rubber motor (14 strands of 1/16-inch rubber for each propeller, total weight 2 oz.) is in position, the centre of gravity will be further forward, in front of the main plane.  The amount of rubber mentioned is for a total weight of 6-1/2 oz.  If the weight of the model alone be 6 oz., you will probably have to use 16 strands, which again adds to the weight, and makes one travel in a vicious circle.  Therefore I lay emphasis on the advice, Keep down the weight.

The front edge of the elevator should be set about 3/8 inch higher than the back, and the model be tried first as a glider, with the rubber and propellers in position.  If it glides satisfactorily, wind up the motor, say 500 turns, and launch by hand.  When a good flight has been obtained, and the correct angle of the elevator has been determined, place the model on a strip of linoleum, wind up, and release the propellers.  The model should rise in its own length and remain in the air (if wound up 900 turns) at least three quarters of a minute.  Choose a calm day if possible.  If a wind blows, let the model face the breeze.  Remember that the model flies high, and select a wide open space.  Do not push the model forward; just release the propellers, held one in each hand near the boss by the fingers and thumb.  As a lubricant for the rubber use pure glycerine.  It is advisable to employ a geared-up mechanical winder, since to make 1,800 turns with the fingers is rather fatiguing and very tedious.

Simple as this model may seem in design, one built by the writer on exactly the lines given has met the most famous flying models of the day in open competition and proved successful against them.

XXVI.  APPARATUS FOR SIMPLE SCIENTIFIC EXPERIMENTS.

Colour Discs for the Gramophone.—­The gramophone, by virtue of its table revolving at a controllable speed, comes in useful for a series of optical experiments made with coloured discs bearing designs of different kinds.