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. Hence sound has in air a certain velocity which has been very
accurately measured, and found to be 1,093 feet per second when the
temperature of the air is at the freezing point of water. As the
temperature increases, the velocity of sound will increase a little more
than one foot for every Fahrenheit degree; so that at 60 deg. the velocity
is 1,125 feet per second. This is the velocity in air. In water the
velocity is about four times greater, in steel sixteen times, in
pine-wood about ten times.