103 
of the sun less active, in the spring and autumn than in the 
winter and summer months ; and in connection with this it 
may be remarked that the rate of change in the difference 
between the temperature of the air and the temperature 
of evaporation is greatest in the months of April and 
September. 
Since the first part of this paper was printed in the 
Society’s Proceedings, Mr. Mackereth, F.RA.S., has com- 
municated to the Physical and Mathematical Section the 
monthly results of his Solar Radiation Observations made at 
Eccles during the five years 1862-66. As these observations 
were made with a black-bulb thermometer placed somewhat 
similarly to that used at Oxford, and as the series extend 
two years beyond the Oxford series, it was evidently very 
desirable to examine how far they confirmed the conclusions 
derived from the Oxford observations. The mean annual 
values of solar radiation having been deduced from the 
monthly means, and Mr. Mackereth having kindly supplied 
me with the annual mean temperatures of the air and of the 
dew-point, they were treated on the plan employed in dis- 
cussing the Oxford observations. These data and the results 
thus derived from them are shown in the following table : — 
Mean 
Temperature 
of Air. 
Mean 
Temperature 
of 
Dew Point. 
Difference. 
Mean 
Amount 
of Solar 
Radiation. 
Ratio. 
1862 
47-3 
42-3 
50 
10-90 
2-18 
1863 
48-3 
42-3 
6-0 
11-47 
1-91 
1864 
47-0 
40’ 5 
65 
11-50 
1-77 
1865 
48-6 
433 
5-3 
9-72 
1-83 
1866 
48-3 
423 
6-0 
8-25 
1-37 
The mean ratio for the two years 1865-6 is 1*60, while 
