We have observed that a species of greatness arises from the artificial infinite; and that this infinite consists in an uniform succession of great parts: we observed too, that the same uniform succession had a like power in sounds. But because the effects of many things are clearer in one of the senses than in another, and that all the senses bear analogy to and illustrate one another, I shall begin with this power in sounds, as the cause of the sublimity from succession is rather more obvious in the sense of hearing. And I shall here, once for all, observe, that an investigation of the natural and mechanical causes of our passions, besides the curiosity of the subject, gives, if they are discovered, a double strength and lustre to any rules we deliver on such matters. When the ear receives any simple sound, it is struck by a single pulse of the air which makes the ear-drum and the other membranous parts vibrate according to the nature and species of the stroke. If the stroke be strong, the organ of hearing suffers a considerable degree of tension. If the stroke be repeated pretty soon after, the repetition causes an expectation of another stroke. And it must be observed, that expectation itself causes a tension. This is apparent in many animals, who, when they prepare for hearing any sound, rouse themselves, and prick up their ears; so that here the effect of the sounds is considerably augmented by a new auxiliary, the expectation. But though after a number of strokes, we expect still more, not being able to ascertain the exact time of their arrival, when they arrive, they produce a sort of surprise, which increases this tension yet further. For I have observed, that when at any time I have waited very earnestly for some sound, that returned at intervals, (as the successive firing of cannon,) though I fully expected the return of the sound, when it came it always made me start a little; the ear-drum suffered a convulsion, and the whole body consented with it. The tension of the part thus increasing at every blow, by the united forces of the stroke itself, the expectation and the surprise, it is worked up to such a pitch as to be capable of the sublime; it is brought just to the verge of pain. Even when the cause has ceased, the organs of hearing being often successively struck in a similar manner, continue to vibrate in that manner for some time longer; this is an additional help to the greatness of the effect. SECTION XII. THE VIBRATIONS MUST BE SIMILAR.