66 Royal Society :— 
Experiment has shown that clouds exert the most powerful influ- 
ence in reflecting the chemical rays; when the sky is partially 
covered by light white clouds, the chemical illumination is more than 
four times as intense as when the sky is clear. Dark clouds and 
mists, on the other hand, absorb almost all the chemically active 
rays. 
“The chemical action of the direct sunlight was determined by 
allowing a known fractional portion of the solar rays to fall perpen- 
dicularly on the insolation vessel of the chemical photometer. The 
solar rays reflected from the mirror of a Silbermann’s heliostat 
were passed through a fine opening of known area into the dark 
room, and a large number of reductions and corrections had to be 
made in order to obtain, from the direct observations, the action, 
expressed in degrees of light, which the sun shining directly upon 
the apparatus would have produced if no disturbing influences had 
interfered. This action of direct sunshine was determined on three 
different cloudless days for various altitudes of the sun. As the sun 
approached the zenith the observed action rapidly increased ; thus 
at 7" 9' a.m., on September 15, 1858, when the sun’s zenith 
distance was 76° 30', the reduced action amounted to 5:5 degrees of 
light, whilst at 9" 14' a.m. on the same day, the zenith distance 
being 58° 11’, the action reached 67°6. This increase in the sun’s 
illuminating power is owing to the diminution in length of the 
column of air through which the rays pass. If we suppose the 
atmosphere to be throughout of the density corresponding to a 
pressure 0°76 and a temperature 0°, and consider it as a horizontal 
layer, and if A represent the action effected before entrance into the 
atmosphere, the action, when the ray has passed through a thickness 
of atmosphere =, is represented by 
W,=Al0-™, 
pale 
where — signifies the depth of atmosphere through which the ray 
has to pass to be reduced to ;/;th of its original intensity, and where 
1 is dependent on the atmosphere’s perpendicular height =A, and 
the sun’s zenith distance ¢. The numerical values of Aa and 7 may 
be calculated from the direct observations, and hence the action W. 
effected at any other zenith distance ¢,, and under a pressure P,, is 
found from the equation 
—ahP, 
W,=A10 % 4 Po, 
where P, represents the atmospheric pressure under which A and a 
are calculated. A comparison between the actions W, thus obtained 
and those, W,, found by experiment, shows as close an agreement as 
could be expected where the observational errors are necessarily so 
large. 
i these experiments it is seen, that if the sun’s rays were not 
weakened by passage through the atmosphere, they would produce 
an illumination represented by 318 degrees of light; or they would 
effect a combination in one minute on a surface on which they fell 
eo 
