ON SOLAR RADIATION. 



149 



least. As the amount of aqueous vapour in the atmosphere 

 is much greater in summer than in winter,, this result 

 tends strongly to confirm the view taken in my paper 

 " On the Theory of Rain/^ and since held to be established 

 by Professor TyndalFs experimental investigations, that 

 air charged with aqueous vapour has a much gi'eater power 

 of absorbing and radiating heat than dry air. If, however, 

 we use the Greenwich observations in this investigation, 

 we shall arrive at a totally different result. Taking, for 

 instance, those made in 1857, ^^^ treating them by the 

 method employed in discussing the Oxford observations, 

 we have the results shown in the following 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 colunm. 



January 



February ... 



March 



April 







9'o 

 i6-6 

 i8-2 

 23-8 

 24-8 

 269 

 26-8 

 24-8 



22'9 



20"2 



9-6 

 3-9 



/ 



17 19 

 25 42 



36 37 

 48 29 



57 31 

 61 52 



59 59 

 52 25 

 41 21 

 29 47 

 19 52 

 15 12 



•38 



•51 



•62 

 •68 



•71 

 •72 

 •72 

 •69 



•05 

 •56 



•43 

 ■33 



23"6 

 32'6 

 29-3 

 35'o 

 34-9 

 37-3 

 37-2 

 35-9 

 35-2 

 36-1 



22'4 



II-9 



May 



June 



July 



August 



September ... 



October 



NoYember ... 

 December ... 



The numbers in the last column of this Table are con- 

 siderably greater in the summer than in the winter months, 

 thus indicating a greatly reduced absorptive action of the 

 atmosphere in the sun^s heating rays in the warmest half 

 of the year, when the quantity of aqueous vapour in the 

 air attains a maximum, a result directly opposed to that 

 derived from the Oxford observations, and to the conclu- 

 sions drawn by Professor Tyndall from his experiments. 

 If, however, we examine the conditions under which the 

 observations were made at Greenwich, we find that while 

 the ordinary thermometers on the shade-stand were placed 



