of Insolation to Atmospheric Humidity. 395 



of great intensity (Schlagintweit, * Meteorology of India,' pp. 49 

 &51). 



To test this point, the results of the observations for the four years 

 ending 1844, over which the inquiry at Madras extended, were di- 

 vided into groups of contemporaneous observations of temperature, 

 vapour-tension, and percentage of clear sky, when the mean nume- 

 rical results showed that a progressive fall of about 1° F. for every 

 •10 of vapour-tension was accompanied by a proportionate increase 

 in the percentage of clear sky, — a result which is the more significant 

 when it is considered that the infusion of visible cloud was limited 

 to -10. 



To prevent mistake, the fall in temperature on the nights of maxi- 

 mum clearness was compared with the fall on nights of minimum 

 clearness within the limits above stated. There were twenty-five 

 nights which were estimated to be perfectly clear, and twenty-two 

 nights when the percentage of clear sky ranged from *90 to '93, the 

 average being *915 (1*0 representing an entirely clear sky). The 

 results were as follows : — 



The mean fall of temperature at an estimated clearness of sky de- 

 noted by 1-0 was8°-3 F. 



The mean fall of temperature at an estimated clearness denoted by 

 •915 was 6° F. 



The difference is 2°*3. The contemporaneous mean tensions of 

 vapour were '68 and -83 respectively. 



It is probable then, since cloud accompanies humidity, that tension 

 of vapour gives some indication of the state of transparency of the 

 sky. 



In any case it is sufficiently clear, both from the results at Madras 

 and from the slight increase in tension at 2 h p.m., that the amount 

 of aqueous vapour alone is not sufficient to account for increased or 

 diminished solar radiation. 



The explanation of the phenomenon of high insolation occurring 

 on days of great relative humidity in India, however, which has been 

 already alluded to, applies with increased force to the absorptive 

 properties of visible moisture ; and the known action of even the light- 

 est form of cloud in radiating heat to the earth, would point to this 

 as the principal cause of the phenomenon, though it cannot be 

 doubted that it is partly due to the action of invisible vapour, whether 

 as warmed directly by the solar rays, or by heat derived from a secon- 

 dary source. 



The dependence of terrestrial heat on vapour and cloud. — It 

 was to increased or diminished radiation under a clear or clouded 

 sky that the inflections of the curves of mean temperature, which 

 were communicated to the Royal Society by the author in May 1865, 

 were ascribed. The effects produced were on that occasion considered 

 to be principally due to radiation at night ; but an examination of the 

 daily means proved that a similar action occurred also by day, — the 

 phenomenon in fact depending, as in the case of increased insolation, 

 on the quantity of moisture in the air. 



