This section is from the book "The Boy Mechanic Vol. 2 1000 Things for Boys to Do", by Popular Mechanics Co.. Also available from Amazon: The Boy Mechanic, Vol2: 1000 Things for Boys to Do.
The body of the horn A is made Of metal, about 3 in. in diameter, with a screw cover. The shaft B, to which is attached a driving pulley, C, and a ratchet wheel, D, Is fitted in holes drilled through the diameter of the body. The diaphragm E is clamped, between the edge of the body and the cover, on a seat made of rubber rings, F, and carries a contact device, G, that is riveted to its center. The diaphragm should be set so that the contact will touch the ends of the ratchets.
A metal cone is fastened in an opening cut in the center of the cover, over the diaphragm. The back of the body is fitted with a bracket for attaching it to the front fork on a bicycle. The lever H carries an idler pulley which is forced against the bicycle tire and the pulley C by means of a cord, J.
Adjusting the diaphragm contact on the ratchet wheel will change the tone of the horn. - Contributed by P. Mertz, Jamaica, L. I.