296 Lord Kelvin on 



of air at the crater of Etna (barometric pressure, 53' 6 centi- 

 metres of mercury) . Hence 



F^T=>-'»-' • • • «k 



and using this in (24) we find 



Here, as in § 57 in connexion with Bouguer's estimate for 

 loss of light in transmission through air, we have an essential 

 uncertainty in respect to the effective wave-length ; and, for 

 the same reasons as in § 57, we shall take \=6 . 10~~ 5 cm. as 

 the proper mean for the circumstances under consideration. 

 With this value of X, (27) becomes 



/(!-*) 



1-63. 10 19 .... (28). 



§ 72. In Mr. SehVs observations on Monte Rosa the zenith 

 distance of the sun was 50°, and the place of the sky observed 

 was in the zenith. He found the brightness of the sun's disc 

 to be about 5000000 times the brightness of the sky in the 

 zenith. Dealing with this result as in §§ 70,^ 71, with /3=0 

 in (20), and supposing the temperature of the air at the place 

 of observation to have been 0° C, we find 



"-^fb] 2 ' 25 - 1019 • • • • (29), 



where e',f, and ri are the values of e, /, and n, at the place 

 of observation on Monte Rosa. Denoting now by JV the 

 number of molecules in a cubic centimetre of air at 0° C. and 

 pressure 75 centimetres of mercury, we have, by the laws of 

 Boyle and Charles, on the supposition that the temperature 

 of the air was 15° on the summit of Etna, and 0° on 

 Monte Rosa 



, 7 75 / n 15 \ ,75 



or iY=l-48ra=l'53n' . ... (30). 



From these, with (28) and (29), we find 



jy= J^J- . 10^= J \ U . 10* . . (31). 



