84 



radiation strikes the ground and neighboring plants and accumuLates 

 therein. 



After three years' use of this instrument, Marie-Davy selected the 

 observations made on the days of perfect clearness of the sky, of 

 which there were only nine, since many days that would be called 

 cloudless showed slight traces of haze. For these days the difference 

 between the readings of the black bulb and bright bulb is represented 

 closely by the exponential formula 



t — f = 17° X 0.875« 

 where the exponent e represents the thickness of the layer of air 

 through which the sun's rays must pass in order to reach the observer ; 

 this thickness, of course, increases as the sun approaches the horizon, 

 being unity for the zenith and 10 for an altitude of 2°, as shown by 

 the following table, which is an abstract of that used by Marie-Davy 

 in his computations : 



Thickness of the hiijer of air traversed liy the solar rays, as computed hy Luin- 

 iert's formula. 



As the formula of Lambert has been chosen by Marie-Davy for use 

 in connection with his form of actinometer, we have therefore given 

 its results in the preceding table ; but as the more accurate formula, 

 as given by Laplace, has been applied to other forms of actinometers, 

 and may even be preferred for the Arago-Davy instrument, I there- 

 fore give a table showing the thickness by the formula of Laplace as 

 used by VioUe and the value of the intensity (I) as given by Violle. 



