We have now considered the effects produced by the wide, and extended agitation, of the air; but there is another kind of agitation, of which the air is susceptible--a vibratory trembling motion, which, striking on the drum of the ear, produces _sound_. _Caroline._ Is not sound produced by solid bodies? The voice of animals, the ringing of bells, the music of instruments, all proceed from solid bodies. I know of no sound but that of the wind, which is produced by the air. _Mrs. B._ Sound, I assure you, results from a tremulous motion of the air; and the sonorous bodies you enumerate, are merely the instruments by which that peculiar species of motion, is communicated to the air. _Caroline._ What! when I ring this little bell, is it the air that sounds, and not the bell? _Mrs. B._ Both the bell, and the air, are concerned in the production of sound. But sound, strictly speaking, is a perception excited in the mind, by the motion of the air, on the nerves of the ear; the air, therefore, as well as the sonorous bodies which put it in motion, is only the cause of sound, the immediate effect is produced by the sense of hearing: for without this sense, there would be no sound. _Emily._ I can with difficulty conceive that. A person born deaf, it is true, has no idea of sound, because he hears none; yet that does not prevent the real existence of sound, as all those who are not deaf, can testify. _Mrs. B._ I do not doubt the existence of sound, to all those who possess the sense of hearing; but it exists neither in the sonorous body, nor in the air, but in the mind of the person whose ear is struck, by the vibratory motion of the air, produced by a sonorous body. Sound, therefore, is a sensation, produced in a living body; life, is as necessary to its existence, as it is to that of feeling or seeing. To convince you that sound does not exist in sonorous bodies, but that air or some other vehicle, is necessary to its production, endeavour to ring the little bell, after I have suspended it under a receiver in the air pump, from which I shall exhaust the air.... _Caroline._ This is indeed very strange: though I agitate it so violently, it produces but little sound. _Mrs. B._ By exhausting the receiver, I have cut off the communication between the air and the bell; the latter, therefore, cannot impart its motion, to the air. _Caroline._ Are you sure that it is not the glass, which covers the bell, that prevents our hearing it? _Mrs. B._ That you may easily ascertain, by letting the air into the receiver, and then ringing the bell. _Caroline._ Very true; I can hear it now, almost as loud, as if the glass did not cover it; and I can no longer doubt but that air is necessary to the production of sound.