364 



Mr. J. P. Harrison on the Relation of [Feb. 14, 



The explanation of the phenomenon of the maxima of insolation oc- 

 curring 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 lightest 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 some of the effect is 

 due to the action of invisible vapour, whether as warmed directly by the 

 solar rays, or by heat derived from a secondary 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 I communicated to 

 the Royal Society in May 1865, were ascribed*. The effects produced 

 were on that occasion considered to be principally due to radiation at night f; 

 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 solar ra- 

 diation, on the quantity of moisture in the air. 



The precise values of heat resulting from the action and reaction of solar 

 radiation and vapour cannot be ascertained until mean numerical values have 

 been deduced from a sufficient number of hourly observations to afford 

 trustworthy data for determination. The present paper merely helps to 

 establish a relation between solar radiation and humidity, and suggests an 

 additional cause for the effects which appear to follow from it. 



* It has since been found that the value of the difference of temperature for fifty 

 years at G-reenwich, due to terrestrial radiation, was very much understated by me, 

 principally from the fact that the mean temperatures for the greater part of the entire 

 period had been corrected by quantities in the Philosophical Transactions which were 

 afterwards found to be erroneous, and have been disused at Grreenwich for several years. 

 All the conclusions which were based specially on the results for fifty years must there- 

 fore be considered as withdrawn. 



The results derived from the more perfect means for periods of seven and eight years 

 are confirmed by the Oxford thermographs, and by almost identical results which I 

 find were obtained by Dr. Madler from fifteen years' observations at Berlin nearly 

 thirty years ago. 



t See also ' Lectm'es on Scientific Subjects,' by Sir John Herschel, 1866, p. 149 (10 

 % British Association Eeports, 1859, p. 198. 



