SOUND. 



a bell and of a trumpet. We may firft fuppofe, for the fake 

 of fimplicity, a fingle feries of particles to be placed only in 

 the fame line with the direftion of the motion. It is ob- 

 vious, that if thefe particles were abfolutely incompreffible, 

 or infinitely elaftic, and were alfo retained in contad with 

 each other by an infinite force of cohefion or of compreffion, 

 the whole feries muft move precifely at the fame time, as 

 well as in the fame manner. But in a fubftance which is 

 both compreffible and extcnfible or expanfible, the motion 

 muft occupy a certain time in being propagated to the fuc- 

 cefGve particles on either fide, by means of the impulfe of 

 the firft particle on thofe which are before it, and by means 

 of the diminution of its prefTure on thofe which are behind ; 

 fo that when the found confifts of a feries of alternations, 

 the motion of fome of the particles will be always in a lefs 

 advanced ftate than that of others nearer to its fource ; 

 while at a greater diftance forwards, the particles will be in 

 the oppofite ftage of the undulation ; and ftill further on, 

 they will again be moving in the fame manner with the firft 

 particle, in confequence of the effeft of a former vibration. 



It is well known, that folid bodies in general are good 

 conduftors of found : thus, any agitation communicated to 

 one end of a beam is readily conveyed to the ear, applied to 

 the other end of it. The motion of a troop of cavalry is 

 faid to be perceived at a greater diftance, by liftening with 

 the head in contaft with the ground, than by attending to 

 the found conveyed through the air ; and we may frequently 

 obferve, that fome parts of the furniture of a houfe are a 

 little agitated by the approach of a waggon, before we hear 

 the noife which it immediately occafions. The velocity, 

 with which impulfes are tranfmittcd by folids, is in general 

 confiderably greater than that with which they are con- 

 veyed by the air. Mr. Wunfch has afcertaincd this by di- 

 reft obfervations on a feries of deal rods clofely united 

 together, which appeared to tranfmit a found inftanta- 

 neoufly, while a fenfible interval was required for its paff- 

 ing through the air. He alfo found that the blow of a 

 hammer on a wall, at the upper part of a high houfe, is 

 heard as if double by a perfon ftanding near it on the 

 ground ; the firft found defcendmg through the wall, the 

 fecond through the air. It appears from experiments on 

 the flexure of folid bodies of all kinds, that their elalticity, 

 compared with their denfity, is much greater than that of 

 the air : thus, the height of the modulus of elailicity of fir 

 wood is found, by means of fuch experiments, to be about 

 9,500,000 feet ; whence the velocity of an impulfe con- 

 veyed through it muft be 17,400 feet, or more than three 

 miles, in a fecond. It is obvious, therefore, that in all 

 common experiments, fuch a tranfmifiion muft appear per- 

 feftly inftantaneous. There are various methods of afcer- 

 taining this velocity from the founds produced under dif- 

 ferent circumftances, by the fubftances to be examined ; 

 and profcftor Chladni has, in this manner, compared the 

 properties of a variety of natural and artificial produftions. 



Sound may be propagated not only in fingle right lines, 

 or ill parallel lines, but ufually the impulfe that occafions 

 it, more efpecially when four.d is tranfmitted through a 

 fluid, fpreads in every direction, fo as to occupy at any one 

 time nearly the whole of a fpherical furface. It is impof- 

 fible, however, that the whole of this furface ftiould be 

 aff^cAed in a fimilar manner by any found, originating from 

 a vibration confined to a certain direftion, fince the par- 

 ticles behind the founding body muft be moving towards 

 the centre, whenever the particles before it are retreating 

 from the centre ; fo that in one half of the furface, the mo- 

 tions may be called retrograde or negative, while in the 

 other they are dired or pofitive : confequently at the fides. 



where thefe portions join, the motions can neither be pofi- 

 tive nor negative, and the particles muft remain at reft : the 

 motions muft alfo become gradually lefs and lefs fenfible, as 

 they approach to the limit between the two hemifpheres. 

 This Itatement may be confirmed by an experiment on the 

 vibration of a body, of which the motion is limited to a j 

 certain direftion ; the found being (carcely audible, when ' 

 the ear is in a direftion precifely perpendicular to that of 

 the vibration. The found, thus diverging, muft always be 

 fpread through a part of a fpherical lui-face, becaufe its ve- 

 locity muft be equal in every direftion ; fo that the impulfe 

 will always move forwards in a ftraighl line paffing through 

 the centre of the fpliere, or the vibrating body. But when 

 a hemifpherical pulfe arrives at the furface of a plane folid 

 obftacle, it is reflefted, precifely in the fame manner that a 

 wave of water is reflefted, and aflumes the form of a pulfe 

 proceeding from a centre at an equal diftance on the oppo- 

 fite fide of the furface. This refleftion, when it returns 

 back perpendicularly, conltitutes what is commonly called 

 an echo (which fee): but in order that the echo may be 

 heard diftinftly, it is necefiary that the reflefting objeft be 

 at a diftance moderately great, otherwife the returning 

 found will be confuted with the original one ; and it muft 

 either have a fmooth furface, or confift of a number of fur- 

 faces arranged in a fuitable form : thus there is an echo not 

 only from a diftant wall or rock, but frequently from the 

 trees in a wood, and fometimes, as it is faid, even from a 

 cloud. 



If a found or a wave be reflefted from a curved furface, 

 the new direftion which it will aflume may be determined, 

 either from the condition that the velocity with which the 

 impulfe is tranfmitted muft remain unaltered, or from the 

 law of refleftion, which requires that the direftion of the 

 reflefted pulfe or wave be fuch as to form an angle with the 

 furface, equal to that which the incident pulfe before 

 formed with it. Thus, if a found or wave proceed from 

 one focus of an clHpfe, and be reflefted at its circumference, 

 it will be direfted from every part of the circumference to- 

 wards the other focus ; fince the diftance which every por- 

 tion of the pulfe has to pafs over in the fame time, in fol- 

 lowing this path, is the lame, the fum of the lines drawn 

 from the foci to any part of the curve being the fame ; and 

 it may alfo be demonftrated that thefe lines form always 

 equal angles with the curve on each fide. (See Ellipsk.) 

 If an elliple be prolonged without limit, it will become a 

 parabola (which fee) ; hence a parabola is the proper form 

 of the feftion of a tube, calculated for coUefting a found 

 which proceeds from a great diftance into a fingle point, or 

 for conveying a found nearly in parallel direftion? to a very 

 diftant place. It appears, therefore, that a parabolic conoid 

 is the beft form for a hearing trumpet, and for a fpeaking 

 trumpet. See Trumpet. 



The decay of found is the natural confequence of its dil- 

 tribution throughout a larger and larger quantity of matter, 

 as it proceeds to diverge every v/ay from its centre. The 

 aftual velocity of the particles of the medium tranfmitting 

 it appears to diminifti fimply in the fame proportion as 

 the diftance from the centre increafes ; confequently their 

 energy, which is to be confidered as the meafure of the 

 ftrength of found, muft vary as the fquare of the diftance ; 

 fo that, at the diftance of ten feet from the founding body, 

 the velocity of the particles of the medium becomes one-tenth 

 as great as at the diftance of one foot ; and their energy, or 

 the ftrength of the found, only one-hundredth as great. 

 Young's Left, on Nat. Phil. vol. i. left. 31. 



The fpace through which found is propagated, in a given 

 time, has been very difterently eftimated by authors who 



have 



