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102 air, when moving continuously and as one body, is creative of hearing. But, owing to its being very diffluent, it gives out no sound, unless that which is percussed be smooth; when this, however, is the case, the air becomes simultaneously one over the surface, as the surface of every smooth body is one. Every sonorous body is so constituted as to set in motion the air which, by continuity, is one up to the hearing, and the hearing is naturally connected with the air; and owing to sound being in the air, the air which is without sets in motion that which is within. An animal, therefore, does not hear everywhere, neither does the air penetrate everywhere; for the part to be set in motion is a living part, and does not everywhere contain air. The air itself, owing to its ready diffusibility, is without sound; but, when precluded from being dispersed, its motion is productive of sound. The air which is within the ears has been so immured as to be immovable; and this in order that the sense may perceive accurately all variations of its movement. It is for these reasons that we are able to hear when in the water, as the water cannot gain access to the congenital air, or pass into the ear through the convolutions; when, however, this does happen, there is no hearing, any more than there is when the membrane of the ear, which is to it what the skin over the pupil is to the eye, is diseased. But proof is afforded whether the hearing is perfect or not, in that the ear