250. =Wind Instruments.=--In wind instruments, as the flute, horn, etc., it is the vibration of the body of air in the instrument which causes the sound. In the common tin whistle or bird-call, Fig. 183, the sound is produced by the vibration imparted to the contained air by the impulse of the breath through the orifice, B. [Illustration: Fig. 184.] 251. =An Analogy.=--The vibration of a sounding body is much like that of a pendulum. The end of the tuning-fork, Fig. 182, on being struck passes to _b_, and in returning passes by the point of rest, A, as the pendulum does, and reaches _a_. So, also, if a string, A B, Fig. 184, be drawn aside to D, as it flies back to C it will by its inertia pass on to E, and so will continue to vibrate back and forth for some time. The same rule also applies to the extent of the vibrations here as in the case of the pendulum, § 209. The quickness of the vibration is not at all affected by its width. The farther the string, A B, is drawn to one side the greater will be the force with which it will return, and hence it will arrive at its position on the other side of the middle line as soon when drawn far away from this line as it would if drawn but little away. The same thing is true of the vibrations or waves of air, though it can not so easily be made plain to you. 252. =How the Sensation of Sound is Produced.=--The vibration of a sounding body is transmitted to the ear ordinarily through the air, and there strikes upon a little drum, a membrane at the bottom of the external cavity of the ear just like a common drum-head. Here the vibration of the air is communicated to this drum, and from this to a chain of very small bones. From the last of these bones it is transmitted to another very small drum, and from this to a fluid in some very complicated passages in the most solid bone in the body. These may be called the _halls of audience_. In the fluid contained in them are spread out the branches of the nerve of hearing, which receive the impression of the vibration, and transmit it to the brain, where the mind takes knowledge of it. Observe that the vibration, transmitted first through the air, then through the drum, then the chain of bones, then another drum to a fluid, stops at the fluid. What is transmitted from this to the brain by the nerve we know not, and so we call it an impression.