99 



rj , , ,0002946 X 29,60 nnrtoono 



Hence m— 1 = i,ooi8 x 1,0375 x 2 9193 = .0002803 



And J:±=L = 57',82. 



The height of an uniform atmosphere is not affected by the 

 variation of the barometer, and therefore, if t represent the 

 height of an uniform atmosphere, the thermometer be- 

 ing 50° 



7 J, l-{- ,00208 ( t— 32) 



I — IX j-^3j^ 



v 

 a very accurate value of — is not required. If we take 



I' = 5 miles and the semidiameter of the earth = 4000 miles 

 — = ,00125. This in fact is sufficiently accurate. 



But it will be more exact to take ^ == 5,095 miles and the 

 semidiameter of the earth = 3979 miles, and then — = 



^j22L = ,00128. 

 3979 



It is evident that the third and fourth terms of the value of 



the refraction in equat. (1) art. 9- cannot be sensibly affected 



by the variation of ;«, and therefore its mean value may be 



used as to these terms. 



Hence substituting for '^^, and — in equat. (1) art. 9. 



Ave have 



Refractions -^-oo^^'-^j:^ x (l_,000l{^-50)) ^-^^^ x 



57^82 tan. ^--A_ x 0",0739 ,^, + 0",000238 ^^ + 



0^0080 tan. ' 6. 



p2 



