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Mr. J. P. Harrison on Solar Radiation. [June 17 



Royal Society in 1867 *, it was shown, from observations of the black-bulb 

 thermometer and Herschel's actinometer, that maximum effects of solar 

 radiation occur at Greenwich, on the average, some weeks after the summer 

 solstice, and about two hours after mid-day, when the atmosphere would 

 appear to be charged with a considerable amount of vapour. 



These results accord with the fact that the highest readings of the solar 

 thermometer are met with in India in districts of great relative humidity f, 

 the explanation of the phenomenon being, as the author ventured to sug- 

 gest in the paper above alluded to, that an increase of insolation is pro- 

 duced by radiation from cloud and visible vapour. 



During the two years which have elapsed since the spring of 1867, 

 whenever the state of the sky and other circumstances permitted, special 

 observations have been made for the purpose of ascertaining with greater 

 certainty the nature of the relation between insolation and humidity. 



Before proceeding to state results, it will afford additional proof that 

 a connexion between the phenomena really exists, if a passage in the 

 appendix to a work by the late Principal of St. Andrews, until very 

 recently overlooked, is quoted in support of the fact. Mr. Forbes, writing 

 some years ago, employs much the same words that were used in the 

 paper above referred to : — " Cloudy weather, if the sun be not itself greatly 

 obscured, apparently increases the effect of solar radiation " J. 



The action, however, does not appear to be confined to days on which 

 there is visible cloud ; for even on cloudless days (so called) very high read- 

 ings of solar radiation seem to be due to the presence of opalescent vapour, 

 which can be easily detected if the hand or some other screen is held for a 

 few minutes before the sun. 



Thus, on May 2, 1868, at l b 30 m , solar radiation appearing to be rela- 

 tively intense, on raising a screen white glare was observed around the 

 sun, and the tint of the sky, which had previously appeared a fair blue, 

 was found, more especially in the south, to be very pale. 



But the most interesting result of this series of observations is the dis- 

 covery that an apparent increase of solar radiation occurs as the sun enters 

 a white cloud of sufficient tenuity to allow free passage for its rays. 



In October 1867, at 2 h , whilst attentively watching a solar thermometer, 

 a sudden rise was observed to take place, upon which, the sun being im- 

 mediately screened, it was found that it had entered the bright border of 

 a cumulus. 



On May 1 1, 1868, at 22 h 40 m , as a very light cloud approached the sun, 

 which was shining in blue sky, the mercury rose 4°, and in 30 seconds 

 3° more as it entered the white cloud. 



On the same day, at 23 h , the reading of the solar thermometer was 101 °F. 

 when the sun was in the midst of cirri, but it fell in 3 minutes 9° when 



* Proc. Roy. Soc, Feb. 1867. 

 t Proc. Eoy. Soc, March 1865. 



\ Travels through the Alps of Savoy, App. III. p. 417. 



