542 Prof. F. Rosetti's Experimental Researches 



contained therein. Before comparing the thermal effect of the 

 rays of the sun with that of hot bodies placed at a short dis- 

 tance from the pile, it is necessary to find out what is the 

 diminution produced by the atmospheric absorption in order 

 to be able to calculate the thermal effect of the solar rays which 

 fall on the face of the pile. To do this we first require to find 

 out the law of absorption of the rays of the sun during their 

 passage through the terrestrial atmosphere. It has been ad- 

 mitted by many physicists that the law is expressed by the 

 formula q = ab e ; 



q being the quantity of heat received in a unit of time by 

 the unit of surface perpendicular to the direction of the 

 rays in a place on the earth's surface, 



a and b being two constants, and 



e a number which represents the thickness of the atmo- 

 spheric stratum traversed by the rays, taking for unity 

 the height of the atmosphere. 



a represents the quantity of heat which in unit time would 

 be received on the unit surface perpendicular to the direction 

 of the rays at the limits of the atmosphere ; and this is the 

 quantity which it is desired to determine. 



b is the proportion of the diminution of this quantity when 

 the rays have traversed in the perpendicular direction the 

 earth's atmosphere; and this is called the coefficient of ab- 

 sorption. 



Violle attempted to determine the hygrometric state of the 

 air ; and he made simultaneous observations at different heights 

 on Mont Blanc. He found that the phenomenon was pretty 

 well represented by the formula 



R + (Z-z)kf 



y =ap — 760 e; 



in which a, p, k are constants ; 



Z, height of the atmospheric stratum beyond which 



the vapour appears to cease ; 

 z, altitude of the place of observation ; 

 /, mean temperature of the vapour contained in the 



strata of air between Z and z ; 

 e, thickness of the stratum of air traversed. 



This is certainly the best method of expressing and estima- 

 ting the law of atmospheric absorption ; but it requires simul- 

 taneous observations at different altitudes, and is not applicable 

 in the case of observations made at one and the same station. 

 I have consequently been obliged to have recourse to other 

 methods. I tried at first to apply the formula q — ab e to the 

 curves obtained in some davs of observations ; but I soon 



