Q. Why can deaf people hear through an ear trumpet?
A. The ear trumpet restrains the spread of the voice, and limits the diameter of the sound-waves; in consequence of which, their strength is increased.
Q. Why are mountains so noiseless and quiet?
A. Because the air of mountains is very rarefied; and as the air becomes rarefied, sound becomes less intense.
Q. How do you know that the rarety of air diminishes the intensity of sound?
A. If a bell be rung in the receiver of an air-pump, the sound becomes fainter and fainter as the air is exhausted, till at last it is quite inaudible.
Q. What is the cause of echo?
A. Whenever a sound-wave strikes against any obstacle (such as a wall or hill), it is reflected (or thrown back); and this reflected sound is called an echo.
The same laws govern echo as light. (See p. [370].)
Q. What places are most famous for echoes?