88 Transactions of the South African Philosophical Society. 



When the relative humidity is greater than 48 per cent, the 

 difference of maxima seems inclined to decrease. The effect is 

 certainly not very definite ; but such as it is it agrees with Abney 

 and Festing's result that, as the air in cooling approaches the point of 

 saturation it begins to exert a considerable absorptive action upon the 

 solar heat.* As it happens, a good number of higher humidity ratios 

 included under 58 per cent, (which actually stands in place of 

 " greater than 55 per cent."), were near the point of saturation. 



TABLE VI. 



Annual Mean Tempeeatuees coeeesponding to Eatios of 

 Eelative Humidity at Noon. 



Eelative Humidity = 



18% 



23% 



28% 



33% 



38% 



43% 



48% 



53% 



58% 



Max. Temp, in the Sun . . 

 Max. Temp, in the Shade. . 

 Difference of Maxima 







141 

 86 

 55 

 39 







139 

 .83 

 56 

 39 







137 

 80 

 57 

 39 



o 



135 



77 

 58 

 40 



o 



133 

 76 

 57 

 42 



o 



132 

 73 

 59 

 42 



o 



129 

 70 

 59 

 41 



o 



131 

 74 

 57 

 43 







128 

 70 

 58 

 44 



It is not easy to see how Tables V. and VI. are to be reconciled ; 

 and more especially so, because if we approximately reduce the 

 values of Table V. to a constant relative humidity by means of 

 Table VI., and reduce the values of Tables VI. to a constant dew- 

 point by means of Table V., no essential change is effected in the 

 differences of maxima beyond making the sequences a little more 

 regular. The following, however, seem to be fair inferences : — 



The decrease in the difference of maxima with increasing dew- 

 point shown in Table V. is caused by the greater absorption by the 

 air of the heat from the direct rays ul the sun. This absorption may 

 be considered for the present as a function of the quantity of mois- 

 ture present, and not of the humid state of the air ; for if a humid 

 air absorb more solar heat than a drier air (say, e.g., a cold air absorb 

 more than a warm air while the dew-point remains constant), then 

 the difference of maxima in Table VI. should decrease, instead of 

 increasing, as the percentage of humidity increases. Eadiation from 

 the black bulb does not impair the general validity of these con- 

 siderations ; for whether this vary with the quantity of moisture or 

 not, it is hardly likely, ceteris paribus, to be more rapid when the 

 dew-point is high than when it is low. Therefore, for the lower 



* Quoted in Hann's " Handbook of Climatology " (Ward's Edition), p. 119. 

 I have not present access to the original. 



