times nearly abolished.

This sense is sometimes depraved, and smells are perceived when no
odorous substance is present; or odours are perceived to arise from
substances, which are very different from those which we perceive in
a sound state.

There are many diseases likewise of the nose, and neighbouring parts,
which cause a depraved sensation; such as ulcers, cancer, caries; a
diseased state of the mouth, teeth, throat, or lungs; or a vitiated
state of the stomach, which sometimes exhales a vapour similar to
that of sulphureted hydrogen. This sense likewise sometimes becomes
depraved from a diseased state of the brain and nerves.

LECTURE VII.
SOUND AND HEARING.

Having in the last lecture examined the senses of taste and smell, I
now proceed to that of hearing. As the sense of smell enables us to
distinguish the small particles of matter which fly off from the
surfaces of bodies, and float in the air, so that of hearing makes us
acquainted with the elastic tremors or impulses of the air itself.

The sense of hearing opens to us a wide field of pleasure, and though
it is less extensive in its range than that of sight, yet it
frequently surmounts obstacles that are impervious to the eye, and
communicates information of the utmost importance, which would
otherwise escape from and be lost to the mind.

Sound arises from a vibratory or tremulous motion produced by a
stroke on a sounding body, which motion that body communicates to the
surrounding medium, which carries the impression forwards to the ear,
and there produces its sensation. In other words, sound is the
sensation arising from the impression made by a sonorous body upon
the air or some other medium, and carried along by either fluid to
the ear.

Three things are necessary to the production of sound; first, a
sonorous body to give the impression; secondly, a medium or vehicle
to convey this impression; thirdly, an organ of sense or ear to
perceive it. Each of these I shall separately examine.

Strictly speaking, sonorous bodies are those whose sounds are
distinct, of some duration, and which may be compared with each
other, such as those of a bell or a musical string, and not such as
give a confused noise, like that made by a stone falling on the
pavement. To be sonorous, a body must be elastic, so that the tremors
exerted by it in the air may be continued for some time: it must be a
body whose parts are capable of a vibratory motion when forcibly
s