while the electricity from a battery may move as slowly as
15,000 or 20,000 miles a second; but this is very largely a matter of
conductors. Its velocity is seldom above 30,000 miles a second on
ordinary telegraphic lines. If the electricity be used to give signals,
as in ordinary telegraphy, the time required varies nearly as the length
of the line, and in any case is a much greater quantity. Prescott in his
work on the telegraph states that "the time required to produce a signal
on the electro-magnet at the extremity of a line of 300 miles of No. 8
iron wire is about .01 seconds, and that this time increases in a
greater proportion than the length of the line; for example, on a line
600 miles in length it amounts to about .03 seconds." He also states
that it varies much with the kind of magnet used, some forms of magnets
being much more sensitive than others for this work.

Wheatstone proved a good many years ago that the duration of the
electric spark was less than one millionth of a second. When a swiftly
moving body can only be seen by an electric spark, or flash of
lightning, it looks as if it were quiescent. Thus a train of cars
rushing along at the rate of forty or fifty miles per hour appears
sharply defined,--even the driving-wheels of the locomotive can be seen
in detail, which is impossible in continuous light,--and all seems to
be standing still. In like manner will the sails of a windmill, which
may be turning at a rapid rate, be seen apparently at rest. This is
because in the short time during which they are illuminated they do not
appreciably move.

I am not aware that any attempt has been made to measure the velocity of
magnetism. If, however, it be a form of motion in ether, it is probable
that the velocity is comparable to the velocity of radiant energy,
light, which is equal to about 186,000 miles a second.

SOUND.

BEFORE explaining the relation that sound has to telephony, it will be
necessary to make quite plain what sound is, and how it affects the
substance of the body through which it moves. If I strike my pencil upon
the table, I hear a snap that appears to the ear to be simultaneous with
the stroke: if, however, I see a man upon a somewhat distant hill strike
a tree with an axe, the sound does not reach me until some appreciable
time has passed; and it is noted, that, the farther away the place where
a so-called sound originates, the longer time does it take to reach any
listener.