d is left
free to vibrate over the other pole of the magnet and not quite touching
it. Each of the reeds is to be tuned to a different pitch, say the 1, 3,
5, and 8 of the scale. These electro-magnets with their attached
vibrators are to be attached each to a resonant box (see p. 93), which
can respond to that particular number of vibrations per second. This is
the receiving instrument. The sender consists of a like set of reeds
tuned to the same pitch, which can be made to vibrate at will by
pressing a key which sends the current of electricity through its
electro-magnet, which makes and breaks the current. Imagine one of these
keys to be pressed down so as to make the circuit complete: the sending
instrument then has one of its reeds, let it be the 1 of the scale, set
in vibration; the intermittent current traverses the whole line, going
through all four of the receiving instruments. Now, we know from the
study of the action of sounding bodies, that only one of the four
receivers is competent to vibrate in consonance with this tone, and this
one will respond; that is, the vibrations are truly sympathetic
vibrations. If, instead of making the 1 of the scale in the
sending-instrument, the 3 had been made, the current would have gone
through all of the receiving instruments just the same as before, but
only one of them could take up that vibratory movement: three of them
would remain at rest, the 3 responding loudly. In like manner, any
number of vibrating reeds in the sending instrument can make a
corresponding number of reeds in the receiving instrument to vibrate,
provided the latter be exactly tuned with the former. Each transmitter
is connected with but a part of the battery, so that several tones may
be transmitted at the same time. If the performer plays a piece of music
in its various parts, every part will be reproduced: thus we have a
compound or multiple telephone. This instrument has been used during
the past winter to give concerts in cities when the performer was in a
distant place.

It has also been used as a multiple telegraph; as many as eight
operators sending messages simultaneously over the same wire,--four in
each direction,--without the slightest interference.

BELL'S TELEPHONE.

Prof. A. Graham Bell of Boston independently discovered the same means
for producing multiple effects over the same wire; but it appears he did
not practically work it out as completely as did Mr. Gray. But while the