If a sufficiently flexible band of metal or a stiff piece of whalebone be fixed at one end in a vice, and then sharply pulled to one side and suddenly let go, a sound results. The same effect is produced when a tight cord or small rope is plucked at and then suddenly released. In each of these cases, if actual movements are not seen, a certain haze which seems to surround the object may be observed. The same can be seen when a tuning-fork is set into action by a bow, a blow, etc. In the case of the fork a graphic tracing (Fig. 36) can be readily taken on smoked paper, thus demonstrating to the eye that vibrations exist, that they occur with perfect regularity and with a frequency that can be measured. [Illustration: FIG. 36 (Tyndall). Illustrates how the vibrations of a tuning-fork are registered on a blackened (smoked) glass. In order that the movements of the fork shall be traced in the form of regular curves, the surface must be kept moving at a definite regular rate.] A similar observation can be made in the case of stringed instruments. If pieces of paper be laid on the strings of a violin, and the bow then drawn across them, the bits of paper will fly off owing to the movements--_i.e._, the vibrations--of the strings. That a force applied at one end of several objects in a line or series causes an obvious effect at the other end, can be well illustrated in a simple way. If a number of individuals stand one behind another in a line, each with his hands laid firmly on the shoulders of the one next to him, and the person at the end be pushed, the force will be conveyed through all the intermediate individuals, and cause the unsupported person at the distant end to move. So is it with the particles of which the air is composed. The movements begun in the drum set up by contact corresponding movements or vibrations in the adjacent air, which ultimately reach the hearing subject's ear, thereby affect his brain, and are accompanied by that change in consciousness which he terms "hearing." It will be observed that these events constitute a chain, and a break anywhere will prevent a sound being heard; there is then, in fact, no sound. Sounds are characterized by _pitch_, _volume_, and _quality_. The _pitch_ is determined by the number of vibrations that reach the ear within a certain time; the more numerous the sound-waves (vibrations) in a second, the higher the pitch. [Illustration: FIG. 37 (Tyndall). Meant to illustrate vibrations. The impulse communicated by the ball pushed from the hand to all the intervening ones causes only the last to actually move bodily.]