the position of the organ in reference to the effluvia, which are the direct subjects of its action, it being placed in the inside of a canal, where the air is continually forced in and out with every breath we draw. The organ is precisely adapted, both in its nature and its place, to its appointed medium of communication with other bodies; nor is this the only mark of design attending it. This sense is frequently a source of gratification; and although it is less keen and powerful in men than in many inferiour animals, it still has power enough to afford much assistance in this respect, that it often warns us of the presence of objects, which experience has found to be injurious to us. The remark has been justly made, that the senses both of taste and smell are of great use in distinguishing bodies, that cannot be distinguished by our other senses. They are peculiarly quick and exact in their judgments, especially in discerning, before we can ascertain it in any other way, the beginning and progress of those changes, which all bodies are constantly undergoing.

But in both of these senses design and utility are discoverable in reference to food in particular. While the sense of smell guards the entrance of the canal for breathing, the sense of taste has its station at the entrance of the alimentary canal. Hence the food, which we consume, undergoes the scrutiny of both; an intentional and beneyolent provision for protecting men and the animal creation generally against the introduction of what would be noxious to them.

CHAPTER FOURTH.

THE SENSE OF HEARING.

§. 158. Organ of the sense of hearing.

FOLLOWING the order, which has been proposed, we are next to consider the sense of HEARING. And in proceeding to the consideration of this subject, the remark is a very obvious one, that we should be unable to hear, if we had not a sense designed for and appropriate to that result. The air, when put strongly in motion, is distinctly perceived by the touch; but no impression, which it could make on that sense, would cause that internal feeling, which is termed a sensation of sound. Our Creator therefore has taken care, that these sensations shall have their own organ; and it is obviously one of precise and elaborate workmanship. The ear is designedly planted in a position, where with the greatest ease it takes cognizance of whatever is going on in the contiguous atmosphere. When we examine it externally, we not only find it thus favorably situated, but presenting a hollowed and capacious surface, so formed as to grasp and gather in the undulations of air, continually floating and in motion around it. Without, however, delaying to give a minute description of the internal construction of the ear, which belongs rather to the physiologist, it will answer our present purpose merely to add, that these undulations are conducted by it through various windings, till they are

brought in a state of concentration, as it were, against the membrane, called the TYMPANUM. It is worthy of notice, that on the internal surface of this membrane, (the drum as it is popularly called,) there is a nerve spread out in a manner analogous to the expansion of the optic nerve at the bottom of the eye. Whether this nervous expansion be indispensably necessary to the result or not, it is certain that a pressure upon or affection of the tympanum by the external air is followed by a new state of the mind, known as the sensation or perception of sound.

§. 159. Nature of sonorous bodies and the medium of the communication of sound.

When we leave the bodily organ, and looking outward inquire still further for the origin of the sensations, which we have by means of the ear, we find them attributable ultimately to the presence and influence of the substances around us. Those undulations of air, which impinge upon the tympanum, and without which there is no sensation of sound, are caused by the vibrations or oscillations of the particles of certain bodies. The material substances which have this quality are termed sonorous, as wood, brass, iron, &c; but it exists in different bodies in very various degrees.

The quality of sonorousness, therefore, in any substance is properly a susceptibility of motion among its own parts. When it is forcibly struck, this motion exists first in itself, and is afterwards communicated to the circumambient air. The movement of the air, which is thus caused, is again communicated, like the concentric waves of water agitated by a stone thrown into it, to other portions successively, till it reaches the ear.

The air accordingly is the medium of communication between the sonorous body, and the tympanum of the ear. It is true, that many solid bodies are good conductors of sound as well as the atmosphere, but as portions of air, through which the vibratory motion must of course pass, are in all cases interposed between that organ and the sounding body, it is not necessary to dwell upon them

here. It is sufficient for our present purpose merely to understand, that there is in every sounding body in the first place a vibratory motion among its own particles from some cause or other; that this vibration or undulation is communicated from the sounding body to the air and from one portion of air to another, till it reaches the organ of hearing. Why the internal sensation should at once follow the completion of this. process is another inquiry, which we do not undertake to explain. We have before us the antecedent and the consequent, the affection of the organ of hearing by an outward impulse, and the new mental state within; but the reason of this invariable connection in two things, that are entirely distinct and different, is a matter beyond our limited comprehension.

§. 160. Varieties of the sensation of sound.

The sensations, which we thus become possessed of by the hearing, are far more numerous than the words and the forms of speech, having relation to them in different languages, would lead us to suppose. It will help to illustrate this subject, if we recur a moment to the sense of TASTE. The remark has somewhere been made to this effect, and probably with much truth, that if a person were to examine five hundred different wines, he would hardly find two of them of precisely the same flavour. The diversity is almost endless, although there is no language, which distinguishes each variety of taste by a separate name. It is the same in respect to the sensations of sound. These sensations exhibit the greatest variety, although their differences are too minute to be separately and distinctly represented by language.

These views will appear the less objectionable, when it is remembered, that sounds differ from each other both in the tone, and in the strength of the tone. It is remarked by Dr. Reid, that five hundred variations of tone may be perceived by the ear, also an equal number of variations in the strength of the tone; making, as he expressly informs us, by a combination of the tones and of the

degrees of strength, more than twenty thousand simple sounds differing either in tone or strength.

In a perfect tone, a great many undulations of elastic air are required, which must be of equal duration and extent, and follow each other with perfect regularity. Each undulation is made up of the advance and retreat of innumerable particles, whose motions are all uniform in direction, force, and time. Accordingly there will be varieties also and shades of difference in the same tone, arising from the position and manner of striking the sonorous body, from the constitution of the elastic medium, and from the state of the organ of hearing.

Different instruments, such as a flute, a violin, and a bass-viol may all sound the same tone, and yet be easily distinguishable. A considerable number of human voices. may sound the same note, and with equal strength, and yet there will be some difference. The same voice, while it maintains the proper distinctions of sound, may yet be varied many ways by sickness or health, youth or age, or any other alterations in our bodily condition, to which we are incident.

§. 161. Manner in which we learn the place of sounds.

It is a fact particularly worthy of notice in respect to sounds, that we should not know, previous to all experience on the subject, whether the sound came from the right or left, from above or below, from a smaller or greater distance. And this will appear the less surprizing, when we remember,. that the undulations of air are always changed from their original direction by the channels and the windings of the ear, before they strike the tympanum. Abundant facts confirm this statement.

Dr. Reid mentions, that once, as he was lying in bed, having been put into a fright, he heard his own heart beat. He took it to be some one knocking at the door,and arose, and opened the door oftener than once before he discovered, that the sound was in his own breast. Some traveller has related, that when he first heard the roaring of a lion in a desert wilderness, not seeing the animal, he did not