84 
radiation strikes the ground and neighboring plants and accumulates 
therein. 
After three years’ use of this instrument, Marié-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 shght 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 
bt ie 01S 15" 
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 Marié-Davy 
in his computations: 
Thickness of the layer of air traversed by the solar rays, as computed by Lam- 
bert’s formula. 
Altitude | Thick- || Altitude! Thick- 
of sun. | ness, e. of sun. | ness, e. 
— ] } 
5 | 5 
0 12.69 | 25 2.30 
2 10.20 || 30 1.96 
8.28 || 40 1.54 
6.83 |) 50 1.30 
5.75 } 60 1.15 
10 4.92 || 70 1.06 
15 3.58. || 80 1.02 
20 2.80 | 90 1.00 
As the formula of Lambert has been chosen by Marié-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 Violle and the value of the intensity (I) as given by Violle. 
