bited at the Centennial Exposition at
Philadelphia, and concerning which Sir William Thompson said on his
return to England, "This is the greatest by far of all the marvels of
the electric telegraph."

The popular impression has been, concerning the telephone, that the
_sound_ was in some way conveyed over the wire. It will be obvious to
every one who may read this, that such is very far from being the case.
The fact is, it is a beautiful example of the convertibility of forces
from one form to another. There is first the initial vibratory
mechanical motion of the air, which is imparted to the membrane carrying
the iron. This motion is converted into electricity in the coil of wire
surrounding the electro-magnet, and at the receiving-end is first
effective as magnetism, which is again converted into vibratory motion
of the iron armature, which motion is imparted to the air, and so
becomes again a sound-wave in air like the original one.

This was the first speaking-telephone that was ever constructed, so far
as the writer is aware, but it was not a practicable instrument. Many
sounds were not reproduced at all, and, according to the report of the
judges at the Philadelphia Exposition, one needed to shout himself
hoarse in order that he might be heard at all.

THE AUTHOR'S TELEPHONE.

For several years past my regularly recurring duties have taken me over
the various subjects treated of in this book, and each one has been
extensively illustrated in an experimental way, and a considerable
number of new pieces of apparatus and new experiments to exhibit their
phenomena have been devised by me.

Among these, I would mention the following:--

1. Measurement of the elongation of a magnetized
bar.

2. A magneto-electric telegraph.

3. An electro-magnetic instrument for
demonstrating the rotation of the earth.

4. The permanent magnetism of the magnetic
phantom.

5. The convertibility of sound into electricity.

6. The induction of a vibrating magnet upon an
electric circuit.

7. The origination of electric waves in a circuit
by a sounding magnet.

8. The discovery of the action of the air in a
sounding organ-pipe.

9. Two or three methods for studying the
vibrations of membranes.

10. Lissajous forks