[ 299 ] 



XLI. Solar Radiation in connexion with Cloud and Vapour. 

 By J. Park Harrison, Esq. 



To the Editors of the Philosophical Magazine and Journal. 



Gentlemen, 



THE results of some observations of solar radiation as af- 

 fected by cloud, which were communicated to the Royal 

 Society at their last Summer Meeting in 1869, having been 

 thought of sufficient interest to be reprinted in " The Proceed- 

 ings of Societies " in your Journal for January, will you permit 

 me to explain that they were intended to be followed by 

 details of experiments with smaller screens, on perfectly cloud- 

 less days ? It was hoped they might have been completed in the 

 autumn of last year, and some progress was made in July ; but 

 the close and continuous examination of the state of the sky up 

 to the very edge of the sun's disk, which was requisite in order 

 to detect the existence of any light form of cloud, proved so 

 trying to the sight, that a complete cessation for some time of 

 all work of this description was found to be absolutely necessary. 



From a few observations made in October and January, how- 

 ever, I believe I may state that the accession of heat from 

 glare in our atmosphere 5° to 8° round the sun, compared with 

 the heat received from direct solar radiation, is very nearly in the 

 ratio 1:3; but this must be taken subject to correction when the 

 sun is in higher altitudes. The experiments were made in these 

 months under very equal conditions, with the solar thermometer 

 raised several inches above short grass and on perfectly calm 

 days*. 



The increase of insolation observed when light clouds approach 

 the sun was shown in the paper above referred to to be very 

 much greater ; whilst diffused light and heat, from white cloud 

 and vapour beyond 20° from the sun, causes no perceptible differ- 

 ence in the readings of an exposed black-bulb thermometer. 



When the relation of insolation to humidity which was 

 brought before the Royal Society by H. v. Schlagintweitf (with- 

 out his being aware that Mr. Forbes had noticed the same thing 

 some years previously at Edinburgh) was found to be supported 

 by observations of the actinometer and solar thermometer at 

 Greenwich, I too hastily concluded that effects which appeared 

 to be undoubtedly connected with cloud and vapour were due 

 wholly to counterradiation of heat received from the bulb of the 

 solar thermometer and the ground generally, as in the case of a 



* The disks used as screens were placed 6 inches from the bulb of a solar 

 thermometer \% inch in diameter. A disk \\ inch diameter, at the above 

 distance, was assumed to be amply sufficient to screen the sun. 

 t Proceedings of the Royal Society, 1865, vol. xiv. p. 111. 



