41«s 



Sir James Dewar 



[Jan. 28, 



hence the effective black body temperature of the sky. This on the 

 occasion just quoted would be - 36° C, or a difference of tempera- 

 ture between earth <aud zenith of nearly 37°. 



Following A. Angstrom's methods in the Bassour expedition, 

 Kimball also determined the amount of water vapour in the air, 

 while observing the sky radiation. When the two sets of results 

 were plotted, the radiation loss to the sky was seen to decrease as the 

 amount of water vapour in the air increased, the rate of decrease 

 being more pronounced with Kimball's results in America than with 

 Angstrom's results in Africa. 



Table II. contains Kimball's mean results covering nearly a whole 

 year, and progressing from lower to higher water vapour pressures. 



Table II. 



The vapour pressures e are given in the first column, with the surface 

 temperature 1\ in the second. The third column R contains the 

 experimental measurement of the calories supplied to equilibrate the 

 absorbent strip. The black body radiation of the earth o-Tj 4 is 

 given in the fourth column (o- being the radiation constant of 

 8*11 x lfr 11 gramme calories per minute per sq. cm.). The ratio 

 R/o- T x 4 , given in the fifth column, indicates the proportionate loss to 

 the sky, and if subtracted from unity gives the proportionate amount 

 received from the sky, the " black body " at earth temperature being 

 unity. The difference of temperature A T between the earth and 



