is drawn under the bristle a straight line will be scratched on the
glass, but if the tuning fork is struck so that the prongs vibrate
back and forth, then the straight line changes to a wavy line and the
type of wavy line depends upon the fork used.

In the phonograph, a diaphragm replaces the tuning fork and a cylinder
(or a disk) coated with wax replaces the glass plate. When the speaker
talks or the singer sings, his voice strikes against a delicate
diaphragm and throws it into vibration, and the metal point attached
to it traces on the wax of a moving cylinder a groove of varying shape
and appearance called the "record." Every variation in the speaker's
voice is repeated in the vibrations of the metal disk and hence in the
minute motion of the pointer and in the consequent record on the
cylinder. The record thus made can be placed in any other phonograph
and if the metal pointer of this new phonograph is made to pass over
the tracing, the process is reversed and the speaker's voice is
reproduced. The sound given out in the this way is faint and weak, but
can be strengthened by means of a trumpet attached to the phonograph.

[Illustration: FIG. 196.--A phonograph. In this machine the cylinder
is replaced by a revolving disk.]

CHAPTER XXX

ELECTRICITY

280. Many animals possess the five senses, but only man possesses
constructive, creative power, and is able to build on the information
gained through the senses. It is the constructive, creative power
which raises man above the level of the beast and enables him to
devise and fashion wonderful inventions. Among the most important of
his inventions are those which relate to electricity; inventions such
as trolley car, elevator, automobile, electric light, the telephone,
the telegraph. Bell, by his superior constructive ability, made
possible the practical use of the telephone, and Marconi that of
wireless telegraphy. To these inventions might be added many others
which have increased the efficiency and production of the business
world and have decreased the labor and strain of domestic life.

[Illustration: FIG. 197.--A simple electric cell.]

281. Electricity as first Obtained by Man. Until modern times the
only electricity known to us was that of the lightning flash, which
man could neither hinder nor make. But in the year 1800, electricity
in the form of a weak current was obtained by Volta of Italy in a very
simple way; and even now our various e