TOTAL DAYLIGHT AT KEW AND P AT?. A. 
559 
Hence we may with certainty conclude that when the disturbing causes of cloud &c. 
are eliminated, the daily maximum of chemical intensity corresponds to the maximum of 
the sun’s altitude, and that the chemical intensity exhibits no sign of a post-meridian 
maximum, as is observed in the measurements of hourly temperature. 
In order to obtain an expression for the relation existing between the sun’s altitude 
and the chemical intensity of total daylight, a much larger number of observations than 
the foregoing must be made at widely differing altitudes, either on the same day or on 
consecutive days. Such a series of observations was made at Heidelberg (see Proceed- 
ings Roy. Soc. No. 81, 1866) on a cloudless day. The relation between the sun’s altitude 
and the chemical intensity as found in these determinations is graphically represented 
in fig. 1A, Plate XXI., and is seen to be a straight line, the abscissse representing the alti- 
tude and the ordinates the corresponding chemical intensity. The formula 
CI a = CI 0 + const X cl 
represents this relation, where CI a signifies the chemical intensity at any altitude ( a ) in 
circular measure, CI 0 the chemical intensity at the altitude 0, and const, is a number 
to be calculated from the observations. That this formula closely represents the rela- 
tion in the case of the Heidelberg observations is seen from the agreement of the ob- 
served with the calculated intensities. 
Chemical Intensity. 
Altitude. 
r 
Observed. 
Calculated from formula. 
7° 15 
0-050 
0-050 
24 43 
0-200 
0-196 
34 34 
0-306 
0-276 
53 37 
0-437 
0-435 
62 30 
0-518 
0-506 
A similar series of observations made at Para (see page 565 of this paper) under a tro- 
pical sun in April last, in the middle of the rainy season, shows that a similar relation 
holds good between the chemical intensity and the sun’s altitude even when the sky is 
not cloudless. 
Chemical Intensity. 
vV _ 
No. of expts. 
Sun’s mean altitude. 
f 
Observed. 
Calculated from formula. 
22 
73° 40 
0-964 
0-959 
11 
60 40 
0-769 
0-800 
11 
49 28 
0-685 
0-666 
10 
22 58 
0-344 
0-338 
This relation is graphically represented in fig. 1 B, Plate XXI. 
Assuming, as we may fairly do, that the same relation between the sun’s altitude and 
chemical intensity holds good at Kew as at Heidelberg and Para, the value of the inten- 
