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.

The material needed for these discs is cardboard, covered with white paper
on one side, or the Bristol board used by artists. The discs on which the
designs are dr