SOUND. 



ium may be put in motion by the lead breath of the mouth, 

 provided the blaUs he often repeated, and keep time exaftly 

 with the vibrations of tlie pendulum, and alio by tlie like 

 art in raifing a large bell. ' 



Dr. Prieftley conftruftcd an apparatus in order to afcer- 

 tain whether the intenfity of found is affefted by any other 

 property of the air in which it is made befides its mere den- 

 fity. The refult of his experiments with different kinds of 

 air was, that the intenfity of found depends folely upon the 

 uenlity of the air in which it is made, and not at all upon 

 any chemical principle in its conltitution. In inflammable 

 air the found of the bell he ufej was hardly to be diftin- 

 guifhed from the fame in a pretty good vacuum ; and this 

 air is ten times rarer than common air. In fixed air the 

 found was much louder than in common air, fo as to be 

 heard about half as far again ; and thi;. air is in about the 

 fame proportion, denfer than common air. In dephlogifti- 

 cated air the found was alfo lenfibly louder tlian in com- 

 mon air. Experiments and Obfervations, 6cc. vol. v. 

 p. 296, Sec. 



But it is not the air alone that is capable of the impref- 

 lions of found, but water alfo, as is manifefl by ilriking a 

 bell under water, the found of which may plainly enough be 

 heard, only not fo loud, and alfo a fourth deeper, accord- 

 ing to good judges in mui'ical notes. And Merlenne fays, a 

 found made under water is of the fame tone or note as if 

 made in air, and heard under water. 



It does not appear that any diredl experiments have been 

 nade on the velocity with which an inipulfe is tranfmitted 

 through a liquid, although it is well known that liquids are 

 capable of conveying found without difficulty. Profelibr 

 Robifon informs us, tliat he heard the found of a bell 

 tranfmitted by water, at the diltance of 1200 feet. It is 

 eafy to calculate the velocity with which found mufl be pro- 

 pagated in any liquid, of which the comprelTibility has been 

 meafured. Mr. Canton has alcertained, that the elafticity 

 of water is about 22,000 times as great a; that of air : it is, 

 therefore, meafured by the height of a column, which is in 

 the fame proportion to 34 feet, that is 750,000 feet ; and 

 the velocity correfponding to half this height is 4900 feet 

 in a fecond. In mercury alfo, it appears from Mr. Can- 

 ton's experiments, that the velocity mufl be nearly the fame 

 as in water ; in fpirit of wine a little fmaller. The experi- 

 ments were made bv fillinir tlie bulb of a thermometer with 

 water, and obferving the efFetts of placing it in an exhaufled 

 receiver, and in condenfed air ; taking care to avoid changes 

 of temperature, and other fources of error. The fluid rofe 

 in the tube wliun the preliure was removed, and fubfided 

 when it was increafed. A flight correftion, however, is 

 neceffary, on account of the expanfion and contradtion of 

 the glafs, which mult have tended to make the clallicity of 

 tiie fluids appear fomewiiat greater than it really was. See 



COMPRKSSIOK. 



The leaft elaftic fubllance that has been examined is per- 

 haps carbonic acid gas, or fixed air, which is confiderably 

 denfer than atmofpherical air, cxpoled to an equal de- 

 gree of prclfurc. The lieight of the atmofphere, fuppofcd 

 to be homogeneous, is, in ordinary circumltances, and at 

 the fea-fide, about 28,000 feet ; and in falling t'lrough half 

 this height, a heavy body would acquire a velocity of 946 

 feet in a fecond. But from a comparifon of tlie accurate 

 experiments of Dcrham, made in the day-time, with tliofe 

 of the French academicians, made chiefly at night, the true 

 velocity of found is about I 130 feet in a fecond; which 

 agrees very nearly with fome obfervations made with great 

 care by profelfbr Piftet. This difference between calcula- 

 tion and experiment, which has long occupied the attention 



of natural philofophcrs, has been in a great meafure removed 

 by the iuggeftion of La Place, who has attributed the effcft 

 to the elevation of temperature, which is always found t<i 

 accompany the aflion of condenfation, and to the depreiTion 

 produced by rarefaftion. When the denfity of the air is 

 changed, while its elaflicity remains unaltered, which hap- 

 pens when it is expanded by heat, or condenfed by cold, 

 the height of the column, and confequently the velocity, 

 will alfo be altered ; fo that for each degree of Fahrenheit's 

 thermometer, the velocity will vary about one part in a 

 thoufand. Bianconi has aftuaily obfervcd this difference of 

 velocity according to the different heights of the thermo- 

 meter, and it may be Ihewn lefs diredtly by means of the 

 founds of pipes ; but it has not been accurately determined 

 whether or not the correilion, on account of the eflFeil of 

 comprefTion in caufing heat, remains unaltered, aithouo-h 

 Bianconi's experiments agree very well with the fuppofition 

 that no material change takes place in this refpeit. The 

 velocity of found mult alfo be, in fome meafure, influenced 

 by the quantity of moiiture contained in the atmofphere : it 

 mufl; be a little diminilhed by cold fogs, which add to the 

 denfity, without augmenting the elallicity ; and increafed 

 by warm vapours, which tend to make the air lighter: and 

 thefe two oppofite flates are probably often produced in 

 fuccelTion, in wind inftruments blown by ths mouth ; the 

 air within them being at firll cold and damp, and afterwards 

 warm and moift. 



In pure hydrogen gas, the velocity of found ought, from 

 calculation, to be more than three times as great as in com- 

 mon air ; but the diilereiice does not appear to liave been fo 

 great in any experiment hitherto made on the founds of 

 pipes, in gales of different kinds. For fuch experiments, 

 the comparative fpecific gravity of the gas may be moil 

 conveniently afcertained by Mr. Leflie's method of obferv- 

 ing the time employed in emptying a veflel through a fmall 

 orifice, by means ot the preflure of an equal column of wa- 

 ter ; according to the Ample theory, the velocities of the 

 gas thus difcharged ought to be in the fame proportion as 

 the refpedtive velocities with which founds would be tranf- 

 mitted by them : and if any variation from this proportion 

 were dilcovered, it mult be attributed to the different de- 

 grees of heat produced by condenfation in the different 

 fluids. Steam, at the temperature of boiling water, is only 

 one-third as heavy as common air ; confequently the velo- 

 city of found in fleam mufl. be nearly three-fourths greater 

 than in air. 



A certain time is always required for the traiifmiflion of 

 an impulfe through a material fubftance, even through fuch 

 fubftances as appear to be the hardell, and the leaft com- 

 preflible. It is demonftrable that all minute impulfes are 

 conveyed through any homogeneous elaftic medium, whether 

 lolid or fluid, with an uniform velocity, which is always 

 equal to that which a heavy body would acquire by falling 

 through half the height of the modulus of elallicity, that is, 

 in the cafe of the air, half the height of the atmofphere, 

 fuppofed to be of equal denfity ; fo that the velocity of 

 found, Daffing through an atmofphere of an uniform elallic 

 fluid, mufl be the f.ime with that of a wave moving on its 

 furface. In order to form a dillindt idea of the manner in 

 which found is propagated through an elaftic fubflance, we 

 mull firll confider the motion of a fingle particle, which, in 

 the cafe of a node, is pulhed forwards, and then either re- 

 mains flatioiiary, or returns back to its original lituation ; 

 but, in the cafe of a mufical found, is coiitimially moved 

 backwards and forwards, with a velocity always varying, 

 and varying by different degrees, according to the nature or 

 quality of ihc tunc : for inftance, differently in the notes: of 



a bell 



