tact with other such air-particles, strike these others, the latter in turn striking yet others, and so on, both a forward and backward movement being set up (oscillation). These particles lie so close together that no movement at all can be detected, and it is only when the disturbance finally reaches the air-particles that are in contact with the ear-drum that any effect is evident.
This phenomenon of sound-transmission may perhaps be made more clear by the old illustration of a series of eight billiard balls in a row on a table: if the first ball is tapped lightly, striking gently against ball number 2, the latter (as well as numbers 3, 4, 5, 6, and 7) will not apparently move at all, but ball number 8 at the other end will roll away. The air-particles act upon each other in much this same fashion, the difference being that when they are set in motion by a vibrating body a complete vibration backward and forward causes a similar backward and forward movement of the particles (oscillation) instead of simply a forward jerk as in the case of the billiard balls.
Another way of describing the same process is this: the vibration of some body produces waves in the air (cf. waves in the ocean, which carry water forward but do not themselves move on continuously), these waves spread out spherically (i.e. in all directions) and finally reach the ear, where they set the ear-drum in vibration, thus sending certain sound-stimuli to the nerves of hearing in the inner ear, and thus to the brain.
An important thing to be noted in connection with sound-transmission is that sound will not travel in a vacuum: some kind of a medium is essential for its transmission. This medium may be air, water, a bar of iron or steel, the earth, etc.
4. The rate at which sound travels through the air is about 1100 feet per second, the rapidity varying somewhat with fluctuations in temperature and humidity. In water the rate is much higher than in air (about four times as great)
