doubted: probably from the latter; hence the found of a cannon is fo vehement, becaufe, the vacuum occafioned by its firing, being nearly perfect, the furrounding air rufhing with great violence, to fupply this vacuum is proportionately agitated. Somewhat like this, in miniature, occurs on re-admiffion of air to the exhaufted receiver of an air-pump.

Do all creatures poffefs the sense of hearing? Certainly not: yet in proportion to their numerous varieties, we may confider it as a sense rarely with-held. Thofe void of fight, are often void of hearing alfo, which feems of little value, where the most extensive sense would be either useless or injurious.

It has been debated whether fish hear: but with regard to the major part of the active kinds, at least, it is confonant to reason to suppose they do, as well because we find organs feemingly adapted to the fenfe, as because violent founds affright, and perhaps hock them. If I mistake not, lobsters are faid to caft their claws at the noise of thunder, as alfo at the report of a great gun, which we know resembles thunder; and a falute is faid to be a frequent threat by men of war to lobfter boats, when purchasing those fish, to insure good ones. I think also, that few fish, if any, appear on the furface of the waves during a thunder-ftorm; and befide what has been said of the firing a mufquet frightening a fhoal of herrings, it is certain that a ftone, &c. thrown into a rivulet, out of fight of fish, will yet difturb and difperfe them. But we are to remember in what found confifts; that it is a pulfation of the furrounding medium, affecting our nerves with fenfations which, tranfmitted to the fenforium, we call found; fuch

PART II.

L1

pulfations

pulfations in water, proportionate to the density of that medium, and to other circumftances, probably affect fish, as aerial pulfations do ourfelves. Water is known to transmit found, fince noife made at a confiderable depth, has been heard in air by thofe who attended to it; as has been experimented by divers ftriking ftones together under water.

As to beafts, we regard them as poffeffing this fense in different degrees; in fome it is extremely acute, and impreffive, and in fact is a fubftitute for thofe exquifite powers of other fenfes, by which other creatures trace their prey. This we have hinted in the lion; we may fay, in general, wherever is voice, is also hearing; birds of fong, therefore, are confidered as poffeffing this fenfe in great perfection, i. e. great accuracy of judgment in determining upon tone. There is another kind of perfection, that relating to distance, which, probably, appertains to birds of prey; and in this, I think it likely, they are unrivalled. It seems scarce probable, that the lion himself can hear fo far as thefe kinds of birds.

But, though we faid fong birds are extremely accurate in determining between tones very little higher, or lower, than each other (as we gather from their modulation of tones with great nicety), yet, probably, the human ear is the most exact judge in this matter. Muficians will diftinguish between tones, which to unpractifed ears are alike, and blind perfons (especially) often afcertain by their precife judgment of founds, many things, perfons, or places, which to thofe lefs attentive require infpection. As inftances of fallacious judgment in birds, Parrots, which are notorious for loquacity, will often talk

to

to themselves, and miftaking their own voices for those of perfons to whom they have been accustomed, will hold down the head, hold out a claw, dance, and do many other things, as directed by their own bidding.

This reminds us of the utility arifing from the providential differences of voice, not only among mankind, but among animals alfo, whereby parents diftinguish their young from among many, and the young diftinguish their parents. What flight, yet determinate, variations among ourselves contribute to this, are probably at least as effectual among animals. No two sheep baa, nor two cows low, exactly alike; nor do hens cackle, or ducks quaack precisely fimilar.

The feat of hearing is fixed near the fenforium, for the fame reasons as fight is, and, like fight, has more than one organ, as well for precifion, and accuracy of information, as for fecurity against its deprivation at a fingle ftroke. Many perfons are deaf with one EAR, but hear with the other.

The principles adopted in conftructing the ear, are not totally different from those adopted in the eye; in both, the feat of the fenfe is deep, and in both the active principle is converged to a focus; but they differ, in that the rays of light are converged directly (becaufe light paffes in ftraight lines); whereas the rays of found are conveyed by (and after) repeated reverberations, becaufe aerial undulations extend all around, and act in every direction. The general conftruction of the ear, therefore, is externally broad, extenfive, and capacious, that it may collect as much found as poffible; but narrowing by degrees, that it may condenfe the found it has collected; not

too fuddenly, that would fhock us, nor too flowly, that were to admit the poffibility of its being often too late, to excite the mind toward fubjects requesting affistance.

Tracing the progress of founds from the external parts of the ear, to where they principally raise fenfations, we find a paffage of unequal dimenfions, leading internally toward the brain; this paffage is lined by a glandular membrane, which furnishes what is called the cerumen or ear-wax, whose use is to prevent the paffage of infects, &c. into the ear, and to keep the external air from injuring it; for as we cannot close our ears, and have no means of excluding offenfive animalcula (as the eye-lids do from the eye), this bitter fubftance answers the purpofe, and that at all times.

Proceeding in the auditory paffage, we find the membrana tympani, or drum of the ear, which is a fine membranous expanfion, extended on a bony ridge, almost circular; this membrane does not entirely close the paffage, but has on one side a fmail aperture covered with a valve. Though highly beneficial to the fenfe, this membrane is not the feat of hearing. Behind it is a cavity, containing three very fmall bones, denominated from their resemblances, as the hammer (malleus), which is extended into the middle of the tympanum, and is at its other end articulated to a bone called the anvil (incus), which is alfo articulated, by the intervention of an exceeding fmall one called orbiculare, to a fourth bone, called the ftirrup (Stapes); these are usually regarded as the fine quâ non of hearing. They tighten, or relax, the membrana tympani without our knowledge, much on the fame principle as the pupil

of

of the eye is enlarged, or narrowed, and much to the fame purpose; by extending the tympanum, increafing the effect of a little found, by relaxing the tympanum, deadening the force of found when too powerful. From the cavity behind the tympanum, is a paffage to the mouth, through which founds are, under certain circumftances, tranfmitted to the auditory paffage, without the interpofition of the tympanum. This cavity alfo contains a portion of air. To the ftapes or stirrup, belongs a muscle, whofe duty is to pull the ftapes off from a fmall hole, which it otherwife covers; this hole, and another near it, leads into a cavity called the veftibule, which leads into other cavities called the labyrinth; comprising the cochlea, which (like a fnail's fhell) forms two turns and a half fpirally, and is divided into two parts by a spiral lamina, or thin divifion. A very delicate membrane lines the cavity of the labyrinth, and is an expanfion of the auditory nerve (as the retina is of the optic nerve), and the primary part of the organ of hearing. There are other openings, and other cavities vifited by nerves, which tranfmit the fenfations they receive from founds to the brain.

What a furprifing ftructure have we been defcribing! What care, not only that weak founds fhould have their effects, but (efpccially) that ftrong founds fhould not be injurious! and this is moftly to be apprehended, as, by their violence, they might burft the tympanum. To guard against this, bombardiers or others, when firing very large cannon, fill the external ear with fomewhat foft to clofe it; but if this fubftance fhould by any means enter the paffage, it is