ACOUSTICS, in physics, is II uttscie nce which instructs us in tile nature of sound. It is divided by some writers into diacous tics, which explains die properties of those sounds that come distinctly from the so norous body to the ear; and catacoustics, hich treats of reflected sounds; but this distinction is not necessary. In the infan c-y of philosophy, SOUI/t1 WaS held to be a separate existence : it was conceived to be wafted through the air to our organs of hearing, which it wassupposed to affect ilk a manner resembling that in which our nostrilsare affected whentheygivens the sensation of smell. Yet, even in those early years of science, there were some, and, in particular, the celebrated founder of the Stoic school, who held that sound, that is, the cause of sound, was only the particular motion of external gross matter, propapited to the ear, and there produc ingthat agitationof the organ, by which the soul is immediately affected with the sensation of sound. Zeno savs, " I leari ng is prothteed by the air which intervenes between the thing sounding- and the ear. The air i3 agitated in a spherical form, and moves off in waves, and falls on the c-ar, in the same manner as water undulates in circles When a stone has been thrown into it." The ancients were not remarkable fur precision, either of conception or ar piment, in their discussions, and they v, ere contented with a g-eneral and vague view ofthings. Some followed the opinion nf Zeno, without any farther attempts to give a distinct conception of the explana tion, or to compare it with experiment. Brit, in latster times, during the ardent re searches into the phenomena a nature, this became an interesting subject of in quiry. The invention of the air-pump gave the first opportunity of deciding, by experiment, whether the elastic undula tions of air were the causes of sound ; and the trial fully established the point ; for a bell rung in vacuo gave no sound, and one rung in condensed air gave a very loud one. It was therefore received its a doc trine in general physics;that air was the velucle of sound. The celebrated flaliko, the parent of mathematical philosophy, discovered the nature of that connection between thelengths of musical chords and the notes which they produced, which bad been observed by Pythagoras, or learned by him in his travels in the East, and which he made the foundation of a refined and beautiful science, the theory of music. Galileo shewed, that the real connection subsisted between the tonesandthe vibra tions of these chords, and that their dif ferent degrees ofacuteness corresponded to the difkrent frequency of their vibra tions. The very elementary and familiar demonstration which he gave of this con nection did not satisfy the curious mathe maticians of that inquisitire age, and the mechanical theory of musical chords was prosecuted to a great degree of refine ment. In the course of this investigation, it appeared that the chord vibrated in a manner precisely similar to a pendulum vibrating- in a cycloid. It must therefore agitate the air contiguous to it in the same manner: and thus there is a particular kind of agitation that the air can receive and maintain, which is very interesting.
Sir Issac Newton took up this question as worthy of his notice ; and endeavoured to ascertain with mathematical precision the mechanism of this particular class of undulations, and gave us the principal theorems concerning the undulations of elastic fluids, which make the 47, 8c.c. Pro positions of Book II. of his Principles of Natural Philosophy. They have been considered as giving the doctrines con cerning the propag-ation of sound. Most sounds, we all know, are conveyed to us by means attic air. In whatever manner they either float upon it, or arc propelled forwanl in it, certain it is, that, without the vehicle of this or some other fluid, we should have no sounds at all. Let the air be exhausted from a receiver, and a bell will emit no sound; for, as the air conti nues to grow less dense, the sound dies away in proportion, so that at last its strong-est vibrations are almost totally si lent. Thus air is a vehicle for sound. However, we must not, with some philo sophers, assert, that it is the only vehicle; that, if there were no air, we should have no sounds whatsoever: for it is found, by experiment, that sounds are conveyed through water with the same facilitv with which they move through air. A beil rung in water returns a tone as distinct as if rung in air. This was observed by Dr. Dertuun, who also remarked, that the tont: came a quarter deeper. It appears from the experiments ofnaturaliscs, that fishes have a strong perception of sounds, even at the bottom of deep rivers. From hence it would seem not to be very material in the propagation of sounds, whether the fluid which- conveys them be elastic or otherwise. Virater, which, of all substan ces that we knOw, has the least elasticity, yet se'rves to carry them forward : and if we make allowance for the.clifference of its density-, Peihaps the sotmds move in it with a proportionable rapiffity to what they are found to do in the elastic fluid of air. But though air and water are both ve hicles of sound, yet neither of them, ac cording th some philosophers, seems to be so by itself, but only? as it contains orals: ceedingly bt le fluid, capable ofpenetrat ing tlie most solid bodies. One thing, how ever, is..certain, that whatever sound ive hear' isaaroduced by.a stroke, which the sounding. bedy Makes against tffe, whether air or water. The fluid, being struck upon, carries the impression for ward to the ear, and there produees,its sensafien, Philosophers are so faragreed, that they all allow that sound is nothing more' than the 'impression made by an elastic bfidy upon the air or.water, and this impression carried along by either fluid to the organ of hearing. But the
mariner in which this convey-ance is made is still 'disputed : whether the solifid is diffused into the air, in circle bekond cle, like the waves of water vihen we dis turb-the. smoothness of jts surfa.ce by' dropping in a stotie ; or whether it travels along, like' rays diffused from a centre, somewhat in the swift manner that- elec. tricity along a rod of iron ; theaaing the questions which have " divided 1111f. learned,- New ton was of the.first opinion. He has explainedthe progression Of sound by an undulatory, or rather a vermicular, motion in the parts of the air. If we have an exact idea of the crawling of some in sects, we shallhave a tolerable notion of the progression of sound upon this hypo thesis. The insect, for instance, in its motion, first carries its contractions from the hinder part, in order to throw its fore part to the proper distance, then it carries its contractions from the fore part to the hinder, to bring that forward. Something' similar to this is the motionof the airwhen struck upon by a sotuidingbody. All who have remarked tbe tone of a be/1,. while its sounds are decaying away-, must have an idea of the pulses of sound, which, ac cording to Newton, are formed by the air's alternate progression and recession. And it must be observed, that as each of these pulses is formed by a single vibration of the string, they must be equal to each other ; for the vibrations of the string known to be so. Again, as to the veloci. ty with which sounds travel, this Newton determines, by the most difficult calcula tion that can be imagined, to be in pro portion to the thickness of the parts of the air, and the distance of these parts from each other. From hence he goes on to prove, that each little part moves back ward and. forward like a pendulum ; and from thence he proceeds to demonstrate, that if the atmosphere were of the same density eveiy where as at the surface of the earth, in such a case, a pendulum, that reached from its highest surface down to the surface of the earth, would, by its vi brations, diseover to us the proportion of the velocity tt.tii which sounds travel. The velocity withwhich each pulse would move, he shows, would be as mach great er than the velocity of such a pendulum swinging with one complete vibration, as the circumference of a circle is greater than the diameter. Front hence he calcu lates that the motion of.sound will be 979 feet in one second.. But this not being, consonant tit experience, he takes in ano ther consideration, which destroys entire ly the rigour of hisffermer demonstration, namely, vapours in pie air, and then finds the motion of sound to be 1 142 feet in one second, or near 13 miles in a minute, a proportion which experience had eSta blished nearly before. 'Many other theo ries on this subject have been advanced by ingenious men, but ,ottr limits do not allow to enter farther • Sinceby-experirnentaithasbeen proved thin sound travels at about the ritte of 1142 feet in a second, and that no obstacles hin der its progress, a contrary, wind only a small matter diminishing its velocity, the nrethod of cal culati ng its progress is easily made knoWn. When a gun is discharged at a distarice, we see the fire long before we hear the sound. If then we know the dist.snce of the Ace, and know the time of the interval between our florstseeing tbe fire aral hearing the report, this will shew us exactly the time that the sound laas been travelling to ,us. For instanee, if the gun is discharge4 a mile oft the mo ment the flash is seen you take a irateh, and count the seconds till you hear the sound, the number of seconds is the time the sound has been travelling a mile. We are also enabled to find the distance between objects that would be otherwise immeasureable. For example; suppose you see the flash of a gun in the night at sea, and tell seven tteconds before you hear the report. it follows therefore that the distance is seven times 1142 feet. In like manner, if you observe the number of se conds between the lightning and the re port of the thunder, you know the distance of the cloud from whence it proceeds. But, according to another philosopher, Dr. Thomas Young, the velocity of sound is not quite so great. " It has been demon strated," he observes, " by M. De La Orange and others, that any impression h atevcr, communicated to one particle of an elastic fluid, will be transmitted th rough that fluid with an uniform veloCity, de-;' pending on the constitution of the fluid, without reference to any supposed laws . of the continuation of that impression. Their theorem for ascertaining this velo city is the same as Newton has deduced from the hypothesis of a particular law of continuation : but it must be confessed, that the result differs somewhat too wide ly from experiment to give us full con fidence in the perfection of the theory. Corrected by the experiments of Various • observers, the velocity of any impression. transmitted by the common air may, at an average, be reckoned 1130.feet in a second." Phil. Trans. vol. XC. • Dr. Derh am has proved by e xperiment, that all sounds whatever tratel at the same" rate. The sound of a grin, and the strik ing of a hammer, are equally swift in their motions ; the softest whisper as swift ly, as far as it goes, as the loudest thun-. der. To these we may add, that smooth and clear sounds proceed from bodies that are homogeneous, and of an uniform figure; and harsh er obtuse solinds,from such as are of a mixed matter and irregu lar figures The velocity of sounds is to that of a brisk wind as fifty The strength of sounds is greatest in cold and dense air, and least in that which is warm and rarefied. Every point against, which the pulsesof soundstrike, becomes, a cen tre, from which a new series of pulsesare propagated in every direction. Sound describes equal spaces in equal times.