lass tube. The top of the rods are hooked, so
that pieces of gold leaf may be suspended from them. A bell-glass is now
placed over the record, table, and rods, and the air is sucked out by a
pump. As soon as a good vacuum has been obtained, the current from the
secondary circuit of an induction coil is sent into the rods supporting
the gold leaves, which are volatilized by the current jumping from one
to the other. A magnet, whirled outside the bell-glass, draws round the
iron armature on the pivoted table, and consequently revolves the
record, on the surface of which a very thin coating of gold is
deposited. The record is next placed in an electroplating bath until a
copper shell one-sixteenth of an inch thick has formed all over the
outside. This is trued up on a lathe and encased in a brass tube. The
"master," or original wax record, is removed by cooling it till it
contracts sufficiently to fall out of the copper mould, on the inside
surface of which are reproduced, in relief, the indentations of the wax
"master."

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