Mrs. B. The weight of the atmosphere is neither increased nor diminished by the ærial tides. The moon's attraction augments the bulk, as much as it diminishes the weight, of the column of air; these effects, therefore, counterbalancing each other, the ærial tides do not affect the barometer.
Caroline. I do not quite understand that.
Mrs. B. Let us suppose that the additional bulk of air at high tide, raises the barometer one inch; and on the other hand, that the support which the moon's attraction affords the air, diminishes its weight or pressure, so as to occasion the mercury to fall one inch; under these circumstances the mercury must remain stationary. Thus, you see, that we can never be sensible of ærial tides by the barometer, on account of the equality of pressure of the atmosphere, whatever be its height.
The existence of ærial tides is not, however, hypothetical; it is proved by the effect they produce on the apparent position of the heavenly bodies; but this I cannot explain to you, till you understand the properties of light.
Emily. And when shall we learn them?
Mrs. B. I shall first explain to you the nature of sound, which is intimately connected with that of air; and I think at our next meeting, we may enter upon the subject of optics.
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?