394 Royal Society ; — Mr. J. P. Harrison on the Relation 



Of the very few observations in May and July which were avail- 

 able for comparison, the scale-readings in July were found to reach far 

 higher values than in May. 



The mean tension of vapour in May 1864 was '30, in July "38. 



The last Table contains, in parts of scale, the results of groups of 

 observations near midday and 2 p.m., on three days, in March, May, 

 and July. The maximum in each case occurs at the later hour. 





iVctinometer-observations at h and 2 h . 



Month and 

 day. 



Mean solar 

 time. 



Mean results 

 in scale-di- 

 visions. 



Altitude of 

 sun. 



State of sky. 



March 24. 

 24. 



h m 



49 



1 32 



14-3 

 17-0 







39 



36 



Clear throughout. 

 Clear. 



Mav 16. 

 16. 



28 

 2 17 



21-4 

 28-3 



56 



47 



Cloudless. 

 Cloudless. 



July 14. 

 14. 



23 37 

 2 13 



48-6 



57-6 



59 

 50 



Cloudless. 

 Cloudless. 



Increased solar radiation supposed to be due to the action of 

 aqueous vapour. — Herr von Schlagintweit, applying Professor Tyn- 

 dall's discovery of the absorptive properties of aqueous vapour to the 

 phenomenon of insolation, attributed the high readings of his solar 

 thermometer, in certain parts of India, to the fact that air, when 

 highly charged with moisture, impedes free radiation ; that is to 

 say, the air restores to the instrument some portion of the heat which 

 has been radiated off from it. 



A like cause has been recently assigned for the variations in tem- 

 perature which take place on clear nights in Madras under different 

 tensions of vapour, those nights being considered clear on which the 

 percentage of cloud did not exceed *10. It was found by Colonel 

 Strachey, on a careful tabulation of hourly observations, that the fall 

 in temperature was decidedly greater when the quantity of vapour was 

 relatively small*. 



Cloud, haze, and opalescence f of the atmosphere more probably 

 the principal cause of the phenomenon. — It appeared of much im- 

 portance to ascertain whether the presence of cloud, and the imper- 

 fect state of transparency in the sky which usually accompanies it, 

 may not have materially assisted in producing the results alluded to 

 in the last paragraph, and, a fortiori, account for the increased inso- 

 lation noticed in cloudy weather in various parts of India — e. g. " on 

 days in the rainy season when the clouds are temporarily broken," 

 and, as in Sikkim, " when a break in the clouds of an hour or two 

 had to be watched for to obtain observations of solar radiation " 



* Phil. Mag. July 1866 ; where see Tables and method of deduction. 



f This term was first used by Professor Roscoe. It here represents the state 

 of the atmosphere at the moment vapour is in process of condensation previously 

 to its formation into cloud at the time of maximum tension. 



