Copies are made from the mould by immersing it in a tank of melted wax. The cold metal chills the wax that touches it, so that the mould soon has a thick waxen lining. The mould and copy are removed from the tank and mounted on a lathe, which shapes and smooths the inside of the record. The record is loosened from the mould by cooling. After inspection for flaws, it is, if found satisfactory, packed in cotton-wool and added to the saleable stock.
Gramophone master records are made on a circular disc of zinc, coated over with a very thin film of acid-proof fat. When the disc is revolved in the recording machine, the sharp stylus cuts through the fat and exposes the zinc beneath. On immersion in a bath of chromic acid the bared surfaces are bitten into, while the unexposed parts remain unaffected. When the etching is considered complete, the plate is carefully cleaned and tested. A negative copper copy is made from it by electrotyping. This constitutes the mould. From it as many as 1,000 copies may be made on ebonite plates by combined pressure and heating.
[32] The Edison Bell phonograph is here referred to.
[33] Some of the sibilant or hissing sounds of the voice are computed to be represented by depressions less than a millionth of an inch in depth. Yet these are reproduced very clearly!
Chapter XVII.
WHY THE WIND BLOWS.
Why the wind blows—Land and sea breezes—Light air and moisture—The barometer—The column barometer—The wheel barometer—A very simple barometer—The aneroid barometer—Barometers and weather—The diving-bell—The diving-dress—Air-pumps—Pneumatic tyres—The air-gun—The self-closing door-stop—The action of wind on oblique surfaces—The balloon—The flying-machine.
When a child's rubber ball gets slack through a slight leakage of air, and loses some of its bounce, it is a common practice to hold it for a few minutes in front of the fire till it becomes temporarily taut again. Why does the heat have this effect on the ball? No more air has been forced into the ball. After perusing the chapter on the steam-engine the reader will be able to supply the answer. "Because the molecules of air dash about more vigorously among one another when the air is heated, and by striking the inside of the ball with greater force put it in a state of greater tension."