The phenomena most analogous to these vibratory affections of the ear, as depending on successive impulses, are unquestionably the phenomena of _titillation_, or rather, to express what is so familiar and simple, by a more homely and appropriate word, the phenomena of _tickling_. In this, the great circumstance distinguishing musical feeling, is to be found, that the feeling arises not from the separate impressions, but from their successions or co-existence. When the palm of the hand is gently tickled, as the finger passes rapidly and repeatedly over the palm, the parts first affected are again affected with various degrees of pressure, as the ear, in melody, is successively affected by repeated varieties of vibration; and various parts of the organ of touch exist, at the same moment, in various states, forming one joint result of sensation, as, in harmony, various vibrations of the organ of hearing _co-exist_, and blend together in one mingled delight. To produce tickling, a certain rapidity of succession is necessary; for, if the parts, first affected, have returned to their original state, before other parts begin to be affected, or themselves to be affected again, the slow motion, it is evident, may be continued, for any length of time, without any effect, different from that of simple pressure. The quicker, then, the return of the parts may be to their original state, the less will be the titillation; and, it is very probably, a difference in this quickness of return, which constitutes the difference of ticklishness, so remarkable in different individuals, who feel, equally, the light pressure of each separate touch. That there is a difference of ticklishness, in different persons, you all know; some being easily excited even to convulsive laughter, by slight motions, that scarcely produce any effect in others, beyond that of the simple primary sensation of touch. A person who is ticklish, and a person who is not ticklish, agree in receiving this first tactual sensation; but they differ afterwards, in this respect, that when the same slight impulse is rapidly repeated, on the same surface, it produces a livelier effect than before, in the _one_, but not in the _other_. The organ of the one who is not ticklish is in the same state, or nearly in the same state, when it receives the second, third, and fourth impression, as when it received the first, and no peculiar excitement, therefore, is produced. The organ of the other, more susceptible, or more tenacious of the affection produced, has not returned to its original state, when the rapid impression is repeated, and is, therefore, at every new impression, affected in a different manner.