560 
PROFESSOR ROSCOE ON THE CHEMICAL INTENSITY OF 
sity at noon can be calculated from the observations at 2 h 30 m and 4 h 30 m p.m. The 
observed values of the monthly mean chemical intensities at 9 h 30 m a.m., 2 h 30 ul p.m., 
and 4 h 30 m p.m., from April 1865 to April 1867, are given in Table III.; the values 
of the intensities at noon have been calculated by help of the foregoing formula. 
Table III. 
Month. 
Hour. 
Mean 
intensity.. 
Month. 
Hour. 
Mean 
intensity. 
1 Month. 
Hour. 
Mean 
intensity. 
1865. 
h m 
1865. 
h m 
1866. 
h m 
April. 
9 30 
0-195 
Dec. 
9 33 
0-029 
August. 
9 34 
0-194 
12 0 
0-297 
12 0 
12 0 
0-280 
2 25 
0-215 
2 26 
0-020 
2 30 
0-210 
4 38 
0-112 
1866. 
4 42 
0-115 
May. 
9 30 
0-211 
January. 
9 34 
0-038 
Sept, 
9 45 
0-172 
12 0 
0-356 
12 0 
12 0 
0-286 
2 21 
0-240 
2 26 
0-047 
2 32 
0-187 
4 30 
0-115 
Feb. 
9 39 
0-051 
4 38 
0-058 
June. 
9 33 
0-192 
12 0 
0-094 
October - . 
9 41 
0-085 
12 0 
0-313 
2 26 
0-065 
12 30 
0-088 
2 26 
0-223 
4 31 
0-021 
2 30 
0-059 
4 39 
0-116 
March. 
9 35 
0-081 
4 34 
0-019 
July. 
9 35 
0-218 
12 0 
0-101 
Nov. 
9 37 
0-042 
12 0 
0-283 
2 30 
0-075 
12 0 
0-057 
2 SO 
0-214 
4 31 
0-041 
2 27 
0-035 
4 30 
0-129 
April. 
9 37 
0-129 
4 21 
0-002 
August. 
9 39 
0-177 
12 0 
0-163 
Dec. 
9 43 
0-028 
12 0 
0-254 
2 31 
0-116 
12 0 
2 28 
0-187 
4 43 
0-057 
2 32 
0-016 
4 44 
0-104 
May. 
9 37 
0-167 
1867. 
Sept. 
9 39 
0-236 
12 0 
0-259 
January. 
9 50 
0-033 
12 0 
0-397 
2 28 
0-164 
12 0 
2 38 
0-271 
4 48 
0-067 
2 31 
0-019 
4 35 
0-106 
June. 
9 43 
0-205 
Feb. 
9 46 
0-042 
October 
9 31 
0-066 
12 0 
0-248 
12 0 
0-080 
12 0 
0-063 
2 33 
0-183 
2 27 
0-053 
2 32 
0-042 
4 43 
0-106 
4 30 
0-012 
4 29 
0-013 
July. 
9 38 
0-229 
March. 
9 53 
0-057 
1 Nov. 
9 37 
0-046 
12 0 
0-330 
12 0 
0-099 
12 0 
2 32 
0-238 1 
2 21 
0-071 
2 29 
0-025 
1 
4 39 
0-141 
4 36 
0-033 
The relations existing between the sun’s altitude and the mean monthly chemical 
intensities are graphically represented (for 1865) in fig. 2, Plate XXI.; and (for 1866) in 
fig. 3, Plate XXI. The ordinates denote the intensity, and the abscissae the correspond- 
ing altitude of the sun. 
From the variation in direction of the straight lines representing the relation of inten- 
sity to altitude for the different months, it is clear that in each month a different value 
exists for the constant of the formula, which in fact represents the degree of atmo- 
spheric opalescence, the amount of cloud, and the various other factors which, in addi- 
tion to the sun’s altitude, influence the chemical intensity. 
That the simple relation which has been shown to hold good when the sun has 
reached a certain altitude does not apply in the case of low altitudes, is distinctly seen 
from the above-mentioned figures. When the sun is only a few degrees above the 
