128 PROPAGATION Ot §0UN15. 



by wMch it is tranfmitted to the ear, they have attended only 

 to its velocity in air, and have employed two different me- 

 thods to determine this, theory and experiment. 

 This velocity Tiiefe two methods have led to the following remarkable 



unifopa in a refults: I ft, that the velocity of found in a given medium, is 



given medium, , /'.,,. , 



unitorra, whatever its diftance from the phonic centre, and 



and the denfity whatever its intenfity: 2dly, that the denfity of the medium 



of the medium ^^ p^.^,^} preffures is one of the elements of the velocity; for it 



IS one of Its ele- , . \. . , , 7 » " 



mcnts. has been found by theory, that the velocity of found is the fame 



as that of a body falhng from half the height of an atmof- 

 phere fuppofed to be of equal denfity with the air in the place 

 where the found is tranfmitted; and by experiment, that, all 

 other circumltances being equal, the velocity of found is the 

 Not afFefted by ^^^^ ^^ different preffures of the barometer; fo that it is equal 

 the height of on the fummit of a mountain and on the fea-Qiore. In fa6l, 

 t e arometer. ^j^^ denfity of the air being proportional to the comprefling 

 weight, the height of the cohimn of mercury in the barometer, 

 divided by the denfity occafioned by this preffure, is a confiant 

 quahtily ; and the height of the atmofphere of a uniform den- 

 fity being equal to the total weight of the air divided by its 

 denfity, it follows that the height of the barometer ought to 

 BlanconI afferts inake no difference in the t^elocity of found. Blanconi afferts 



it to be lefs in (Comment. Bonon^ vol. 11. p. 365), that the velocity of found 

 Vfjnter than in . , ,. . ..., , ,V ■ r r j' . i • 



fummer. *^ lets in Winter than m lunimer, fince, according to hisexpe- 



' riments, it takes four feconds more in winter to traverfe a fpace 



D<^rhatn denies of fixteen Halian miles, Derham affirrtis, that the velocity of 



thisv bu^ is pro- ^ ^^ -g the fame whether the air be extremely hot or extremely' 

 •ably wrong. ^ •/ ^ 



cold, though his tables of experiments will be found on ex- 

 amination favourable to the opinion of Blanconi; for thd 

 greatefl velocity of found in them was on the5lh of April, at 

 one o'clock in the afternoon, being three miles in 111 half-' 

 feconds, and the leaft velocity on the 12lh of February, at fi:iZ 

 o'clock in the evening, being three miles in 122 half-feconds. 

 As the experiments on the velocity of found undertaken by the 

 Academy of Sciences in 1737 w^ere made at temperatures that 

 exhibit only two or threcdegrees difference, perhaps it would 

 be well, as Mr. Laplace thinks, if they were repeated at a- 

 time of the year when the temperature is very different; foi^j 

 Kot affeftrd by experience has taught us, that this velocity is equal in rainy 

 rain or fine ^^^^ j^ ^^^^ weather, fo that nothing but change of temperature 

 can prc^lucc any variation in this refpe^. 

 .. T ■ ' • Wha 



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