The necessity of hearing to man, consid ered as a social being, is obvious; its impor lanceto him, considered as a being whose pleasures and pains are by depress to be purely mental, is not inferior. The means of knowledge are greatly diminished by the loss of sight ; but the loss of sight only impedes the progress of the mind from sensation to thought and feeling. Those who have never heard have much greater disadvantages to undergo. Their deficiencies can never be fully supplied. Words, as Hartley suggests, are highly important, and even necessary, to the full improvement of intellect, and the en largement of the affections ; and there fore the car is of much more importance to us, as spiritual beings, than the eye. To illustrate the cause of sound, it is to be observed, 1st. That k motion is ne cessary in the sonorous body for the pro duction of sound. 2dly, That this mo tion exists first in the small and insensi ble parts of the sonorous bodies, and is excited in them by their mutual collision against each other, which produces the tremulous motion so observable in bodies that have a clear sound, as bells, musical chords, &c. idly, That this motion is communicated to, or produces a like mo tion in the air, or such parts of it as are fit to receive and propagate it. Lastly, That this motion must be communicated to those parts that are the proper and immediate instruments of hearing. Now that motion of a sonorous body, which is the immediate cause of sound, may be owing to two different causes; either the percussion between it and other hard bodies, as in drums, bells, chords, &c. or the beating and dashing of the sonorous body and the air immediately against each other, as in flutes, trumpets, &c. But in both these cases, the motion, which is the consequence of the mutual action, as well as the immediate cause of the sonorous motion which the air con verys to the ear, is supposed to be an in visible, tremulous, or undulating motion, in the small and insensible parts of the body.
The sonorous body having made its im pression on the contiguous air, that im pression is propagated from one particle to another, according to the laws of pneumatics. A few particles, for instance, driven from the surface of the body, push or press their adjacent particles into a less space ; and the medium, as it is thus rarefied in one place, becomes condens ed in the other; but the air thus com pressed in the second place is, by its elasticity, returned back again, both to its former place and its former state and the air contiguous to that is compress ed ; and the like obtains, when the air less compressed, expanding itself, a new compression is generated. Therefore, from each agitation of the air there arises a motion in it, analogous to the motion of a wave on the surface of the water, which is called a wave or undulation of air. In each wave, the particles go and return back again through very short equal spaces; the motion of each particle be ing analogous to the motion of a vibrat ing pendulum, while it performs two oscillations ; ind most of the laws of the pendulum, with very little alteration, be ing applicable to the former.
Sounds are as various as are the means that concur in producing them. The chief varieties result from the figure, constitution, quantity, &c. of the sono rous body ; the manner of percussion, with the velocity, &c. of the consequent vibrations ; the state and constitution of the medium ; the disposition, distance, &c. of the organ ; the obstacles between the organ and the sonorous object and the adjacent bodies. The most notable distinction of sounds arising from the various degrees and combinations of the conditions above mentioned, are into loud and low (or strong and weak ;) into grave and acute (or sharp and fiat, or high and low ;) and into long and short. The management of which is the office of music.
Euler is of opinion, that no sound making fewer vibrations than 30 in a se cond, or more than 7520, is distinguish able by the human ear. According to this doctrine, the limit of our hearing, as to acute and grave, is an interval of eight octaves. The velocity of sound is the same with that of the aerial waves, and does not vary much, whether it go with the wind or against it. By the wind indeed a certain quantity of air is carried from one place to another ; and the sound is accelerated while its waves move through that part of the air, if their direction be the same as that of the wind. But as sound moves vastly swifter than the wind, the acceleration it will receive is but inconsiderable ; and the chief effect we can perceive from the wind is, that it increases and diminishes the space of the waves, so that by help of it the sound may be heard to a greater distance than otherwise it woukl.
That the air is the usual medium of sound, appears from various experiments in rarefied and condensed air. In an un exhausted receiver, a small bell may be heard to some distance ; but when ex hausted, it can scarce be heard at the smallest distance. When the air is con densed, the sound is louder in proportion to the condensation or quantity of air crowded in; of which there are many instances in Hauksbee's experiments, in Dr. Priestley's, and others. Besides, sounding bodies communicate tremors to distant bodies; for example, the vibrat ing motion of a musical string puts others in motion, whose tension and quantity of matter dispose their vibrations to keep time with the pulses of air, propagated from the string that wasstruck. Galileo explains this phenomenon, by observing, that a heavy pendulum may be put in motion by the least breath of the mouth, provided the blasts he often repeated. and keep time exactly with the vibrations of the pendulum ; and also by the like art in raising a large bell.
It is not air alone that is capable of the impressions of sound, but water also ; as is manifest by striking a bell under water, the sound of which may plainly enough be heard, only not so loud, and also a fourth deeper, according to good judges in musical notes. And Mersenne says, a sound made under water is of the same tone or note as if made in air, and heard under the water. The velocity of sound, or the space through which it is propaga ted in a given time, has been very differ ently estimated by authors who have writ ten concerning this subject. Roberval states it at the rate of 560 feet in a second; Gassendus at 1473 ; Mersenne at 1474; Duhamel, in the History of the Academy of Sciences at Paris, at 1338 ; Newton at 968; Dcrham, in whose measure Flip stead and Halley acquiesce, at 1142. The reason of this variety is ascribed by Derham, partly to some of those gentle men using strings and plummits instead of regular pendulums ; and partly to the too small distance between the sonorous body and the place of observation ; and partly to no regard being had to the winds. But by the account since published by M. Cassini de Thury, in the Memoirs of the Royal Academy of Sciences at Paris, 1738, where cannon were fired at various as well as great distances, under many varieties of wepther, wind, and other circumstances, and where the measures of the different places had been settled with the utmost exactness, it was found that sound was propagated on a medium, at the rate of 1038 French feet in a second of time. But the French foot is in proportion to the English as 15 to 16: and consequently 1038 French feet are equal to 1107 English feet. Therefore the difference of the measures of Derham and Cassini is 35 English feet, or 33 French feet, in a second. The medium velocity of sound, therefore, is nearly at the rate of a mile, or 5280 feet, in 44 se conds, or a league in 14 seconds, or 13 miles in a minute. But sea miles are to land miles nearly as 7 to 6 ; and there fore sound moves over a sea mile in 5k seconds nearly, or a sea league in 16 seconds. Further, it is a common observa tion, that persons in good health have about 75 pulsations, or beats of the ar tery at the wrist, in a minute ; conse quently, in 75 pulsations, sound flies about 13 land miles, or 11,k sea miles, which is about 1 land mile in 6 pulses, or one sea mile in 7 pulses, or a league in 20 pulses. And hence the distance of objects may be found, by knowing the time employed by sound in moving from those objects to an observer. For ex ample . On seeing the flash of a gun at tea, if 54 beats of the pulse at the wrist were counted before the report was beard, the distance of the gun will easily be found by dividing 54 by 20, which gives 2.7 leagues, or about 8 miles. See Acorsrics.
Seem), in geography, denotes in ge neral any strait, or inlet of the sea, be tween two head lands. However, the name sound is given by way of eminence, to the strait between Sweden and Den mark, joining the German Ocean to the Baltic, being about four miles over.
Sousa board, in an organ, is a reser voir, into which the wind, drawn in by the bellows, is conducted by a port-vent, snd hence distributed into the pipes plac ed over holes in its upper part: this wind enters them by valves, which open by pressing upon the stops or keys, after drawing the registers, which prevent the air from entering any of the pipes, except those it is required in.