538 Prof. F. Rosetti's Experimental Researches 



degrees marked on the instrument. When the air contained 

 an excessive amount of moisture the index went beyond 100°. 



Table IV. 



Hygrometer. 



Relative humidity. 



60 0-361 



65 0-405 



70 0-450 



75 0-500 



80 0-550 



85 0-596 



90 0-646 



Barometric observations were also made ; but we shall see 

 that these indications, as well as those of the hygrometer, can- 

 not be used except in cases of simultaneous observations made 

 at different altitudes. Before giving the tables of observations, 

 it may be useful to show how the value of e, representing the 

 thickness of the atmospheric stratum traversed by the rays of 

 the sun, has been calculated. If at the time of the observa- 

 tion the sun is not more than 60° from the zenith, the formula 

 6= sec Z gives us the value nearly enough. When the sun is 

 lower, it is necessary to calculate e by a formula which gives 

 exact results. At the Mont Souris observatory Pouillet's for- 

 mula is employed ; but when the distance from the zenith is 

 great, this gives too small a value for e. The values which I 

 have entered in the following Tables are calculated from the 

 formula of Bouguer and Laplace, which gives exact and con- 

 cordant results. 



In order to obtain the values quickly, I constructed a curve, 

 the abscissae of which represent the zenithal distances, and the 

 ordinates the corresponding values of e. In order to trace the 

 curve I used the values contained in the following Table, 

 which have been calculated by Forbes with the formula of 

 Laplace. 



Table V. 



Distance from 





Distance from 





zenith. 



e. 



zenith. 



e. 



Z. 





Z. 





o 







1000 







75 



3-809 



10 



1016 



80 



557 



20 



1065 



85 



10-22 



30 



1156 



86 



12-2 



40 



1-306 



87 



14-9 



50 



1-555 



88 



18-9 



60 



1.995 



89 



251 



70 



2-902 



90 



35 5 



