22 Scientific Proceedings, Royal Dublin Society. 
his conclusion. It is confirmed by the observations of meteoro- 
logists on the greater or slower rate of cooling of the Harth at 
night, when the air contains less or greater amounts of water- 
vapour.’ 
In Hann’s “Handbook of Climatology” (Prof. R. de C. 
Ward’s edition) some previous numerical values of mine are 
quoted ; but the final verdict is not exactly in accordance with 
them :—“ The more water-vapour the air contains, the more 
readily do light fogs form, although these may be invisible to the 
eye. It is therefore still uncertain whether the effect of the vapour- 
pressure here shown is to be attributed altogether to the vapour- 
contents of the air. Woeikof believes that the connexion between 
radiation and relative humidity is much more clearly proved. 
Sutton has recently attempted to show the effect of the water- 
vapour in the air upon nocturnal cooling by means of some obser- 
vations at Kimberley, South Africa. For this purpose, he selected 
the observations made on perfectly clear nights during an entire 
year. No relation between the dew-point at 8 p.m. and the 
amount of nocturnal cooling could be established; but that there 
was a marked relation between the relative humidity and the 
nocturnal cooling is shown by the following figures :— 
RELATIVE Humipiry anp Nocrurnat TEMPERATURES AT 
é KIMBERLEY. 
RetativE Humipity at 8 p.m. (Percentages). 
25-39 | 40-49 | 50-59 | 60-69 70-80 and above. 
Mean Dirrerencre oF TEMPERATURE BETWEEN 8 P.M. AND THE Minimum. 
10°38 C. | 8°°8 | 7°°6 | ie? | 6°-0 
1J. H. Poynting and J. J. Thomson, ‘‘ A Text-Book of Physics,’’ ‘‘ Heat,’’ 1904,. 
p- 236. 
