SOUND. 



©nly tliofe next the Turface communicate with the air, 

 the motion of the whole, or of the greater parts, being no 

 farther concerned, than as they contribute to the other. 



To apply this theory, ftrlke a bell with any hard body, 

 and you eafily perceive a fenfible tremor on the furface, 

 fpreadin^ itfelf over the whole, and that more fenfibly as 

 the ihock is greater. Upon touching it in any other part, 

 the motion and the found too are flopped. Now this is ap- 

 parently a motion of the fmall and infenfible parts, changing 

 their fituation, with refpeft to one another ; which being fo 

 many, and fo clofely united, we cannot perceive their mo- 

 tions feparately and diftinftly ; but only a trembhng, which 

 we reckon to be the effecl of the confufion of an infinite 

 number of little particles, clofely joined, and only moving 

 in infinitely little lines. 



M. Perrault adds, that the vifible motion of the parts 

 contributes no otherwife to found, than as it caufes the invi- 

 fible motion of the fmaller parts, which he ca\h particles, to 

 diftinguifh them from the fepfible ones, which he calls 

 parts, and from the fmallefl of all, which we call corpufcUs. 



This he fupports from the inftance of a chord or Itring, 

 which being ilnick, and the found, and fenfible undulations, 

 at reft again, if you approach the chord foftly with the 

 finger, you will find a fmall tremulous motion, which is the 

 remains of the vibrations of the whole firing and the parts. 

 Now the parts vibrate without any found ; but no fooner is 

 the vibration felt by the finger, than the found is heard 

 again ; which he afcribes to this, that the motion of the 

 parts being infufficient to m.ive the particles, whofe motion 

 is the firll that ceafes, requires fome aHiflance from dafhing 

 againll the finger, whereby to become enabled to give the 

 particle the motion necefiary for the producing of found. 

 He finiflies his proof by the inftance of flutes, which, when 

 made of different matters, as wood, metal, &c. whofe parts 

 are very different, but their particles nearly the fame, if their 

 lengths and bore be the fame, there is very little fenfible dif- 

 ference in then- found?. 



The fonorous body having made its impreflion on the con- 

 tiguous air, that impreflion is propagated from one particle 

 to another, according to the laws of pneumatics. 



A few particles, for inftance, driven from the furface of 

 the body, drive their neighbouring particles into a lefs 

 fpace ; and the medium, as it is thus rarefied in one place, 

 becomes condenfed in the other ; but the air, thus comprefled 

 in the fecond place, is, by its elafticity, returned back again, 

 both to its former place and its former ftate ; and the air con- 

 tiguous to that is comprefTed ; and the like obtains, when 

 the air lefs comprefled expanding itfelf, a new comprefTion is 

 generated. From each agitation of the air, therefore, there 

 arifes a motion of the an-, analogous to the motion of a 

 wave on the furface of the water ; which we call a <wave, 

 or undulation of air. 



In each v/ave the particles go and return back again, 

 through very (liort, but equal fpaces ; the motion of each 

 particle is analogous to the motion of a vibrating pendulum, 

 while it performs two ofcillations ; and moll of the laws of 

 the pendulum, with very little alteration, are applicable 

 thereto. 



The only condition neceffary for the produftion of a fim- 

 pie found, is a fufficient degree of velocity in the motion or 

 impulfe which occafiono it. A very moderate velocity muft 

 be fufiicient for producing an impreflion on the ear : and 

 when the found is continued, it may remain audible with a 

 velocity of no more than one hundredth part of an inch in a 

 fecond, or perhaps with a velocity much lefs than this ; but 

 at its origin, it )s probable that the velocity of the motion 



conltituting a found muft always be confiderably greater. 

 A continued found may be produced by a repetition of fe- 

 parate impulfes, independent of each other, as when a wheel 

 ftrikes in rapid fuccelTion the teeth of a pinion, fo as to force 

 out a portion of air from between them ; when a pipe, 

 through which air is pafTing, is alternately opened and fhut, 

 either wholly or partially, by the revolution of a flop-cock 

 or valve ; or when a number of parallel furfaces is placed at 

 equal dillances, in aline nearly perpendicular to them, and j|| 

 a noife of any kind is reflected from each of them in fuccef- fl 

 fion : — a circumftance which may often be obferved, when 

 we are walking near an iron railing, an acute found being 

 heard, which is compofed of fuch refleftions from the fur- 

 faces of the palifades. See Sound, in Muftc, infra. 



Sounds are as various as are the means that concur to 

 their produdlion. The principal varieties refult from the 

 figure, conftitution, quantity, &c. of the fonorous body ; 

 the manner of percuffion, with the velocity, &c. of the 

 vibrations confequent thereon ; the ftate and conftitution of 

 the medium ; the difpofition, diftance. Sec. of the organ ; 

 the obftacles between the organ and the fonorous objeft, and 

 the adjacent bodies. The moil notable diflinftion of founds, 

 arifing from the various degrees and combinations of the 

 conditions mentioned, are into loud and h'w (or ftrong and 

 weak) ; \w\.o grave and acute (or (harp and flat, or high and 

 low); and into long znd. Jbort : the management of which 

 makes the office of mufic. See the next ai^ticle. 



M. Euler is of opinion, that no found, making fewer vi- 

 brations than 30, or more than 75.20 in a fecond, is diftin- 

 guifliable by the human ear. According to this doftrine, 

 the limit of our hearing, as to acute and grave, is a>n inter- 

 val of eight oftaves. Tentam. Nov. Theor, Muf. cap. i. 

 feft. 13. 



The velocity of found is the fame with that of the aerial 

 waves, and does not differ much, whether it go with the 

 wind or againfl it. By the wind, indeed, a certain quan- 

 tity of air is carried from one place to another ; and the found 

 is accelerated, while its waves m.ove through that jxirt of 

 the air, if their direftion be the fame as that of the wind. But 

 as found moves much more fwiftly than wind, the acceleration 

 it will hereby receive is inconfidcrable ; and the chief effcA 

 we can perceive from the wind is, that it increafes and di- 

 minifhes the fpace of the waves, fo that by help of it the 

 found may be heard to a greater diftance than otherwife it 

 would. See Wind, cs" infra. 



That the air is the ordinary medium of found, and that 

 the prcfence of this, or of fome other material fubftance 

 for its tranfmiffion is neceffary, appears from various expe- 

 riments in rarefied and condenfed air. In an uncxhaufted 

 receiver, a fmall bell may be heard at fome diftance ; but 

 when exhaufted, it can fcarcely be heard at the fmallefl 

 diftance. If the air be condenfed, the found will be louder 

 proportionably to the condenfation, or quantity of air 

 crowded in, of which we have many inilances in Mr. 

 Haukfbee's experiment". From thefe experiments (Phil. 

 Tranf. 1709, xxiv. 1902, 1709, xxvi. 367. 371.) it appears, 

 that a bell was heard at the diftance of 30 yards, when the 

 air was in its common ftate, at 60 with the force of two at- 

 mofpheres, at 90 with the force of three : beyond this the 

 intenfity did not much increafe. 



Befides, founding bodies communicate tremors to dittant 

 bodies ; e. g. the vibrating motion of a mufical firing puts 

 others in motion, whofe tenfion and quantity of matter dif- 

 pofe their vibrations to keep time with the pulfes of air, 

 propagated from the itring that was flruck. Gahleo ex- 

 plains this phenomenon by obferving, that a heavy pendu- 

 lum 



