sounds by receiving isochronous vibrations, and reflecting them, shews, that there are latent active powers in the small parts of bodies, which dispose them to vibrate. Now, if there be such powers in the biggest component particles, analogy inclines one to expect them in the several descending orders.
The doctrine of sounds does also furnish us with an answer to one of the principal and most obvious difficulties attending the supposition, that all sensation, thought, and motion, is performed by vibrations in the medullary substance. For it may be objected, that such a number of different vibrations as seems to be required in certain cases, can scarce exist together in the medullary substance. Thus it is not uncommon for a person to receive a series of sensations, carry on a train of thought, and perform a course of external actions, which have little connexion with each other, at the same time. Now to this we may answer, that vibrations as different from each other do, in fact, exist together in common air, in such a manner as to be perceived distinctly. Thus a person may listen to what part he pleases in a concert of music, and masters in the art can listen to more than one. They can also at the same time receive, attend to, and understand the vibrations of the air, arising from the discourse of other persons. But in whatever manner these different vibrations can exist together precisely at the same instant, or rather succeed each other at infinitesimal intervals, without confusion; in the same manner may vibrations and vibratiuncles exist together, or succeed each other, without confusion also, in the medullary substance. And by whatever power the soul is qualified to attend to, and distinguish from each other, these several vibrations of the air, by the same power may it correspond to the vibrations in the medullary substance, so that each shall have its peculiar effect of producing the appropriated sensation, thought, and motion. It is to be observed farther, that there is a difficulty in performing both the things here mentioned; that confusion does often arise; and that where any person is remarkable for doing more than one thing at once, it is in consequence of great practice, and also of exquisite mental powers, i.e. of an exquisite make of the medullary substance, according to the theory of these papers.
As all moderate and tolerably uniform sounds please young children, and the original pleasures from concords sounded together, from the succession of both concords and discords, and even from clear musical sounds, considered separately, remain with us through the whole progress of life, it is evident, that
