In considering sound relatively to its external cause, we give the name of vibration to the successive pulses, or alternate approaches and recessions of the particles of the elastic sounding body; and the word is a very convenient one for expressing this series. But still it may be necessary to warn you, that the word, though single, is not the less expressive of a plurality of states, which have no other unity, than as they are comprehended in this single word,--a word, like many other single words, by which we express the combination of various objects, or invented by us, merely to aid our weakness, that is incapable, without such helps, of conceiving or remembering even a small part of that wide series of physical changes, which we are able to discover in the universe, if each event of the series were to be distinguished by a peculiar name. This mere aid of our weakness, however, we are apt, by a very absurd, but a very general fallacy, to consider as something, much more dignified in its nature than a mere arbitrary verbal abbreviation,--as truly an explanation of the very phenomena, or series of phenomena, which it simply designates. You must not flatter yourselves, however, that you have advanced the slightest step, in explaining the connexion of sound with the pulses of air, when you have merely invented a brief term for those successive pulses, and ascribed the sound to vibration; you have, indeed, given a name to a series of corpuscular phenomena, but you have not discovered any thing additional to the phenomena themselves, which can be considered as explanatory to the changes produced.