: 85 
Thickness of the layer of air traversed by the direct solar rays as computed by 
Laplace’s formula, and the corresponding value of I, the absolute intensity of 
direct sunshine in calories per minute per square centimeter which fall nor- 
mally on any surface through the purest air, as given by Voille. 
F Intensity | : Intensity 
Thickness . .| Thickness Zs 
|Altitude| (e),La- | of di | aitituae| (ce), La- | (of di 
of sun. place shine, || fsun. | _ place aniae 
formula. eaey ||| | formula. wien, 
Violle. Violle. 
0 35.50 0.359 || 30 1.995 2.275 
Ne 18. 90 0. 896 35 1.740 2. 306 
4 12. 20 1.293 | 40 | 1.555 2.831 
6 8. 60 1.540 45 1.420 2.349 
8 6. 85 130) | 50 | 1.305 2. 364 
10 5. 70 1. 868 60 1. 155 2.383 
15 3.81 2.059 70 1. 065 2.395 
20 2.90 2.164 80 1.016 2.401 
25 2.425 2. 229 90 1.000 2.403 
Observation shows that no two such Arago-Davy actinometers 
placed side by side will give exactly the same results; therefore the 
rule has been adopted of comparing all instruments with the stand- 
ard kept at Montsouris, and a standardizing factor is thereby obtained 
by which the observed difference between the bright and black bulb 
of any pair is to be multiplied in order to reduce it to a common 
standard. 
In addition to the standardizing factor of the preceding paragraph, 
Marié-Davy has also introduced the conception of an ideal standard 
actinometer, graduated in such a way that the first factor, 17° in 
the above-given formula as expressed in centigrade degrees, shall 
be represented by 100 ‘“ actinometric degrees” in his ideal instru- 
ment; that is to say, all the differences (¢-¢’) observed with any 
actinometer, after being multiplied by the standardizing factor, have 
still to be multiplied by the factor 5.88 in order to convert them into 
ideal actinometric degrees. For convenience both these factors may 
be replaced by one, and in this way the instrument and Marié-Davy’s 
methods have been extensively employed in studying the relation 
between sunshine and crops. 
In such study Marié-Davy and his pupils take the “sum of the 
total number of actinometric degrees” as the datum for comparison 
with crop reports, instead of the sum of the temperatures of the air 
observed in the shade, or the sum of the soil temperatures as used 
by other investigators. If we divide the actinometric degrees given 
in any case by the factor 5.88 we shall obtain the excess of the black 
bulb over the bright bulb as originally observed in centigrade 
degrees. From this we can obtain the true relative quantities of solar 
radiation by a modification of the method given by Ferrel (pp. 41-50 
