86 



Transactions of the South African Philosophical Society. 



maxima between sun and shade is high when the dew-point is low, 

 the one rising as the other falls. Now this result is brought about 

 by the circumstance that both the sun and shade temperatures rise 

 as the dew-point rises, but that the latter rises almost twice as fast 

 as the former. If we suppose the daw of the Table between dew- 

 points of 28° and 48° to continue, this would make the temperatures 

 in sun and shade equal at something less than 200°, while the corre- 

 sponding dew-point would be about 270°. In general a dew-point 

 higher than the temperature is, of course, out of the question ; but it 

 is curious that the mean of the three, when the first two coincide, 

 should not greatly differ from the boiling-point. This would mean 

 that an atmosphere of aqueous vapour at barometric pressure would 

 absorb the whole of the solar heat. At the absolute zero, by the 

 same law, the difference of maxima would be 325° F., the tempera- 

 ture in the shade being - 459°, and in the sun - 134°. No such 

 supposition, however, is admissible, considering the nature of the 

 evidence and the irregularity of the values in the Table.* 



TABLE V. 



Annual Mean Temperatures corresponding to Assigned Dew- 

 points at Noon. 



Dew-points 



Max. Temp, in the Sun. . 

 Max. Temp, in the Shade 

 Difference of Maxima .... 

 Humidity % 



28° 



33° 



38o 



43° 



48° 



o 



















131 



134 



135 



137 



136 



71 



76 



78 



80 



81 



60 



58 



57 



57 



55 



30 



29 



29 



32 



38 



53° 



137 

 79 



58 

 38 



* Prof. Langley concluded from his observations on Mt. Whitney that in the 

 absence of any atmosphere the temperature of the earth's surface would rise 

 to — 373° F. under direct sunshine. (" .Researches on Solar Heat," p. 123.) But 

 Prof. Poynting shows that according to Kurlbaum's determination of the amount 

 of energy issuing from a fully radiating surface at any temperature, a black sphere 

 1 sq. cm. in cross-section placed in full sunshine at the earth's distance from the 

 sun would attain a surface temperature of 70° F. ; while a flat surface facing the 

 sun would reach 140° F. The earth's surface would probably be 20° less, because 

 it reflects some of the heat. (Nature, Sept. 22, 1904.) It will be remembered that 

 Sir John Herschel thought that the surface of the full moon must necessarily be 

 very much heated, "possibly to a degree much exceeding that of boiling water " 

 (" Outlines of Astronomy," 1851, p. 261). R. A. Proctor held the same view, and 

 quoted with approval Lord Rosse's result that the diurnal range of temperature on 

 the moon amounts to fully 500° F. ("Old and New Astronomy," 1892, p. 523). 

 C. A. Young, however, remarks that " there is no air-blanket at the moon's surface 



