lses that come over the
wire; the varying pull of the magnet sets the diaphragm in motion,
and that sets the air in motion in waves precisely like those of the
distant voice. When those waves strike the listener's ear, he _seems_
to hear the speaker's exact tones, and so, substantially, he does
hear them. The circumstance that electric waves, and not sound-waves,
travel over the wires, does not change the quality of the resulting
sound in the least.

I think you now understand Bell's telephone.

The telephones of Edison, Gray, and others, involve different
principles and are differently constructed.

One invention very often leads to another, and the telephone already
has an offspring not less wonderful than itself. It is called the
speaking-phonograph. It was invented by Mr. Edison, one of the
gentlemen, just mentioned.

Evidently, Mr. Edison said to himself: "The telephone hears and
speaks; why not make it write in its own way; then its record could
be kept, and any time after, the instrument might read aloud its own
writing." Like a great genius as he is, Mr. Edison went to work in the
simplest way to make the sound-recorder he wanted. You know how the
diaphragm of the telephone vibrates when spoken to? Mr. Edison took
away from the telephone all except the mouth-piece and the diaphragm,
fastened a point of metal, which we will call a "style," to the center
of the diaphragm, and then contrived a simple arrangement for making
a sheet of tin-foil pass in front of the style. When the diaphragm is
still, the style simply scratches a straight line along the foil. When
a sound is made, however, and the diaphragm set to vibrating, the mark
of the style is not a simple scratch, but an impression varying in
depth according to the diaphragm's vibration. And that is how the
phonograph writes. To the naked eye, the record of the sound appears
to be simply a line of pin points or dots, more or less close to each
other; but, under a magnifier, it is seen to be far more complicated.

Now for the reading. The impression on the foil exactly records the
vibrations of the diaphragm, and those vibrations exactly measure the
sound-waves which caused the vibrations. The reading simply reverses
all this. The strip of foil is passed again before the diaphragm,
the point of the style follows the groove it made at first, and
the diaphragm follows the style in all its motions. The original
vibrations are thus exactly reproduced, setting