That which is conveyed into the brain by the ear is called sound; though, in truth, till it come to reach and affect the perceptive part, it be nothing but motion.
The motion, which produces in us the perception of sound, is a vibration of the air, caused by an exceeding short, but quick, tremulous motion of the body from which it is propagated; and therefore we consider and denominate them as bodies sounding.
That sound is the effect of such a short, brisk, vibrating motion of bodies from which it is propagated, may be known from what is observed and felt in the strings of instruments, and the trembling of bells, as long as we perceive any sound come from them; for as soon as that vibration is stopped, or ceases in them, the perception ceases also.
The propagation of sound is very quick, but not approaching that of light. Sounds move about 1140 English feet in a second of time; and in seven or eight minutes of time they move about one hundred English miles.
OF SMELLING.
Smelling is another sense, that seems to be wrought on by bodies at a distance; though that which immediately affects the organ, and produces in us the sensation of any smell, are effluvia, or invisible particles, that, coming from bodies at a distance, immediately affect the olfactory nerves.
Smelling bodies seem perpetually to send forth effluvia, or steams, without sensibly wasting at all. Thus a grain of musk will send forth odoriferous particles for scores of years together, without its being spent; whereby one would conclude that these particles are very small; and yet it is plain that they are much grosser than the rays of light, which have a free passage through glass; and grosser also than the magnetic effluvia, which pass freely through all bodies, when those that produce smell will not pass through
