82 Transactions of the South African Philosophical Society. 



in the air, the greater will be the absorption of the sun's heat by the 

 air, and therefore the temperature of the air will be raised at 

 the expense of the heat which would otherwise have reached the 

 black bulb. The evidence of Table II., so far as it goes, says just 

 the opposite. Later on we shall have reason to suspect that the 

 effect which Stow thought to be due to water vapour may be partly 

 due to other causes ; and also, incidentally, that a mere tabulation 

 of monthly averages cannot unravel the tangle of influences that go 

 to the making of a black-bulb temperature. 



TABLE II. 



Comparative Meteorological Elements for the Four Years 



1900-1903. 



January . 

 February , 

 March . . . 

 April 



May 



June 



July 



August . . , 

 September 

 October . . , 

 November. 

 December , 

 Year 



Tempera- 

 ture ia the 

 Sun. 



152-0 

 153-0 



143- 



135- 



127- 



118- 



119- 



126-9 



135-4 



134- 



148' 



151- 



137- 



Difference 

 between 



Maxima in 



Sun and 



Shade. 



Cloud 

 XI. and XIV. 



61-8 



63 



60 



58 



54 



53 



53-0 



55-6 



58-2 



60-1 



62-2 



62-1 



58-6 



% 

 36 

 41 

 37 

 32 

 20 

 22 

 20 

 17 

 34 

 36 

 30 

 40 

 30 



Dew-point 

 at Noon. 



49-3 



53-1 



52-1 



49-2 



40-8 



37-2 



36- 



37- 



40- 



42- 



42-9 



50-0 



44-3 



Humidity 

 at Noon. 



% 

 30-7 

 35-0 

 42-0 

 43-9 

 37-6 

 43-0 

 41-0 

 35-0 

 32-8 

 31-6 

 27'3 

 31-8 

 36-0 



Again, according to an obscure paper hfr J. Park Harrison, H. von 

 Schlagintweit arrived at the conclusion that the maximum isolation 

 came on days of great relative humidity. * This maybe so in India; 

 if we could trust Table II. we should say that it probably is not 

 so in South Africa. For we get both high and low solar temperatures 

 and temperature-differences when the relative humidity is high ; and 

 also high and low humidities when the temperatures and temperature- 

 differences are high. In particular we get a mean monthly tempe- 



* J. P. Harrison, "Note on Solar Radiation in Relation to Cloud and Vapour " 

 Q. J. Mel. S., October, 1875, p. 455. The author does not clearly distinguish 

 between relative and absolute humidity, nor define whether by insolation he means 

 the maximum temperature as registered by the black bulb in vacuo or its excess 

 over the maximum temperature in the shade, 



