Aqueous Vapour on Terrestrial Radiation. 



67 



pension over the whole area traversed by the current of air ; and 

 such variations will of course have affected the final results noted. 

 Further, the means of determining the clearness of the sky are 

 very imperfect, and important differences of transparency may 

 occur of which it is impossible to judge by the eye alone, the 

 register of the state of the sky being a mere eye estimate. 



I have thought that the records for the four years, which I 

 have taken at random, will be sufficient to explain the method 

 adopted, and to show the character of the support given in this 

 way to Dr. TyndalFs views. I may add, however, that just the 

 same results are shown in all other cases in which I have been 

 yet able to examine such observations of night temperatures 

 under a clear sky ; and the following figures are got from obser- 

 vations made between the 4th and the 25th of March, 1850 

 (also taken from the Madras records), during which period the 

 sky remained remarkably clear, and great variations of the quan- 

 tity of vapour took place. 



1 



Tension of vapour 



inch. 

 •888 



•849 



•805 



•749 



•708 



•659 



•605 



•554 



•435 



Fall of temperature 1 

 from 6.40 p.m. to I 

 5-40 a.m J 



Fahr. 



o 



60 



o 



7-1 



8-8 



8-5 



10-3 



12-6 



121 



o 



13-1 



16-5 



The temperature of air at 6 h 40 m p.m. varied from 78° # 9 to 

 84° -7 Fahr. 



I before observed that the variation of the heating-power of 

 the sun prevented useful comparisons between the increase of 

 heat during the day at different times of the year. But the fact 

 comes out clearly enough that the air, when dry, is more freely 

 traversed by the sun's heat than when damp. The observations 

 of March 1 850, arranged so as to bring out this point, give the 

 following results : — 



Tension of vapour .... 



inch. 



•824 



•737 



•670 



•576 



•511 



•394 



Rise of temperature 1 

 from 5.40 a.m. to I 

 to 1-40 p.m J 



Fahr. 

 12-4 



151 



19°3 



22 2 



24-3 



270 



It may be remarked that a possible cause of error in the con- 

 clusions drawn from the nocturnal fall of temperature, namely 

 the greatly increased depression of temperature of evaporating 

 surfaces in dry air, is clearly shown not to be of weight by the 

 diurnal rise of temperature being exaggerated in the dry air 



F2 



