99 
sun attains his greatest meridian altitude, and the minimum in 
December, when his meridian altitude is least. As the 
amount of aqueous vapour in the atmosphere is much greater 
in summer than in winter, this result tends strongly to con- 
firm the view taken in my paper “ On the Theory of Rain/’ 
and since held to be established by Professor Tyndall’s experi- 
mental investigations, that air charged with aqueous vapour 
has a much greater power of absorbing and radiating heat 
than dry air. If however we use the Greenwich observa- 
tions in this investigation we shall arrive at a totally differ- 
ent result. Taking, for instance, those made in 1857, and 
treating them by the method employed in discussing the 
Oxford observations, we have the results shown in the fol- 
lowing table : — 
Mean Amount of 
Solar Radiation 
on Clear Days. 
Meridian 
Altitude of the 
Sun on the 15th 
of the Month. 
Proportion of 
Light 
Transmitted. 
| Number in 
First Column 
divided by 
Number in 
Third Column. 
January 
90 
17 19 
•38 
236 
February 
166 
25 42 
•51 
326 
March 
18-2 
36 37 
•62 
29-3' 
April 
23-8 
48 29 
•68 
35-0 
May 
24-8 
57 31 
*71 
34-9 
June 
26-9 
61 52 
•72 
373 
J uly 
26*8 
59 59 
•72 
372 
August 
24-8 
52 25 
•69 
359 
September 
229 
41 21 
05 
35-2 
October 
20-2 
29 47 
•56 
361 
November 
96 
19 52 
•43 
22-4 
December 
39 
15 12 
•33 
119 
The numbers in the last column of this table are consider- 
ably greater in the summer than in the winter months, thus 
indicating a greatly reduced absorptive action of the atmo- 
sphere in the sun’s heating rays in the warmest half of the 
year, when the quantity of aqueous vapour in the air attains 
