.1825.] Velocity of Sound. 275 



The mean altitude of barometer corrected of the effect of 

 capillarity, and reduced to the temperature of 0° of centrigrade 

 scale, was as follows : 



Station of Zevenboompjes 0'",7439 



Kooltjesberg 7456 



Mean altitude of barometer 74475 = p. 



The mean tension of aqueous vapour in the atmosphere, as 

 determined by Mr. Daniell's hygrometer, was at 



Station of Zevenboompjes = 0,00901235 metres. 

 Kooltjesberg = 0,00949378 

 Mean tension of aqueous vapour, 0,00925307 = f. 



The effect of gravity, calculated for mean latitude of Amers- 

 foort and Naarden, by the formula 



g _ (g) (1 _ 0,002837 cos. 2 /) 



= ^^4su\ 1- ^'^^2837 cos. 2 {52^ 13^ 33^,35} } 



g = 9812,03 = effect of gravity in lat. 52° 13' 33^^35. 



The ratio of the specific heat of the air when the volume 

 is constant, to the specific heat of air at a constant pressure, 



or -, is, according to the experiments of Gay Lussac and Wel- 



ter, equal to 1,3748 = -. 



In Sir Isaac Newton's formula \/ ^, by which the velocity 



of sound is expressed, D is the density of air, that of mercury 

 being taken for unit. 



By Biot's and Arago's experiments, the density of perfectly 

 dry air was found at 0™,76 barometrical pressure to be equal to 

 unity divided by 10466,82. 



But when the barometrical pressure alters and becomes p, 

 and the temperature becomes t, we have by the law of Mariotte 



D 



10466,82 X 0V6{1 +« . 0,00375} 



And introducing into this formula the correction for the 

 aqueous vapour existing in the air, and calling F the tension of 

 aqueous vapour existing in the air, we find 



D = 



p -IF 



10466,82 X 0">,76{1 +<• 0,00375} 



This value of I) being substituted in Sur la^ac's formula, we 

 have the velocity of sound by theory 



t2 — rr-- 



