y of the atmosphere, and
the movement of the winds also changes its power of transmission.

Sound is also made more intense if a sonorous body is near its source.
This is taken advantage of in musical instruments, where a
sounding-board is used, as in the case of the piano, and in the violin,
which has a thin shell as a body for holding the strings.

Another curious thing is shown in the speaking tube, where the sound
waves are confined, so that they are carried along in one line, and as
they are not interfered with will transmit the vibrations to great
distances.

VELOCITY OF SOUND.--The temperature of the air has also an effect on the
rate of transmission, but for general purposes a temperature of 62
degrees has been taken as the standard. The movement is shown to be
about 50 miles in 4 minutes, or at the rate of 1,120 feet per second.

In water, however, the speed is four times greater; and in iron nearly
fifteen times greater. Soft earth is a poor conductor, while rock and
solid earth convey very readily. Placing the ear on a railway track will
give the vibrations of a moving train miles before it can be heard
through the air.

SOUND REFLECTIONS.--Sound waves move outwardly from the object in the
form of wave-like rings, but those concentric rings, as they are
called, may be interrupted at various points by obstacles. When that is
the case the sound is buffeted back, producing what is called echoes.

RESONANCE.--Materials have a quality that produces a very useful result,
called _resonance_, and it is one of the things that gives added effect
to a speaker's voice in a hall, where there is a constant succession of
echoes. A wall distant from the speaker about 55 feet, produces an
almost instantaneous reflection of the sound, and at double that
measurement the effect is still stronger. When the distance is too short
for the reflecting sound to be heard, we have _resonance_. It enriches
the sound of the voice, and gives a finer quality to musical
instruments.

ECHOES.--When sounds are heard after the originals are emitted they tend
to confusion, and the quality of resonance is lost. There are places
where echoes are repeated many times. In the chateau of Simonetta,
Italy, a sound will be repeated thirty times.

SPEAKING TRUMPET.--This instrument is an example of the use of
reflection. It is merely a bell-shaped, or flaring body, the large end
of which is directed to the audience. The voice talking into the sma