90 Transactions of the South African Philosophical Society. 



divisible into two classes : one being absorbable by aqueous vapour, the 

 absorption of the other depending upon the humidity of the space 

 containing the aqueous vapour. There is, however, an alternative 

 and probably a better view, namely, that the solar radiation is 

 absorbable in proportion to the absolute humidity alone (leaving 

 out of account, of course, the case when the condensation limit 

 is approached) ; but that once absorbed it is emitted in rays of 

 lower order and different character, absorbable in proportion to the 

 relative humidity alone. This latter view agrees better with the 

 fact that both reflected solar heat and radiated dark heat reach the 

 earth from the moon.* 



It is interesting to compare the average of the monthly values of 

 the absolute maximum temperature in the sun and the average 

 of the monthly values of the greatest difference of maxima in sun 

 and shade with other meteorological elements of the same days. 

 In Table VII. the first line gives the mean of the absolute maxima, 

 while the second line gives the mean of the greatest difference of 

 maxima between sun and shade. 



TABLE VII. 



Average Values of (1) all the highest Temperatures in 

 the Sun in each Month ; and of (2) all the greatest 

 differences of temperature between sun and shade j 

 compared with other meteorological elements during 

 Three Years. 



Maxima 



in the 



Sun. 



Maxima 

 in the 

 Shade. 



Diffe- 

 rence of 

 Maxima. 



Cloud. 



Relative 

 Humid- 

 ity. 



Dew- 

 point. 



Dust. 



Thunder 

 and 

 Light- 

 ning. 



Hoar 

 Frost or 

 Dew at 



VIII. 







149-3 

 144-0 



o 



85-7 

 75-1 



o 



63-6 

 68-9 



% 

 41 

 54 



% 

 28 

 41 







44-7 

 45-5 



Times. 



12 



3 



Times. 

 13 

 11 



Times. 



7 

 7 



The average black-bulb temperature in the second line is so high 

 because it occasionally happens that the greatest difference of 

 maxima in a given month goes with the absolute maximum tem- 

 perature in the sun. The dew-point differs but little in the two 



* For some interesting remarks bearing upon this see W. M. Davis on " The 

 Absorption of Terrestrial Kadiation by the Atmosphere," in Science, October 11, 

 1895 ; also the Earl of Kosse, " On the Radiation of Heat from the Moon," Phil. 

 Trans., 1873. 



