Mr. J. P. Harrison on Solar Radiation. 71 



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, 1 868, at ] h 30 m , solar radiation appearing to be 

 relatively 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 

 discovery 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 1 867, at 2 h , whilst attentively watching a solar thermo- 

 meter, a sudden rise was observed to take place, upon which, the 

 sun being immediately 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 mi- 

 nutes 9° when well free again ; then rose 6° as light cloud again 

 crossed it. The air was perfectly still. 



On May 15 3 1868, the highest reading of the solar thermometer 

 for the day occurred at 2 h 1 7 m , just as the sun entered the skirts of 

 a cloud. 



On July 21, 1868, at 2 h , the maximum of the day (128° F.) was 

 reached when the sun was shining in a patch of pale sky surrounded 

 with white cumuli, some of which were within one or two diameters 

 of its disk. 



To mention one more example amongst numerous others which 

 might be cited : on Aug. 3, 1868, at h 40 m , under an apparently 

 clear sky, the solar thermometer registering 112°, and the tempera- 

 ture of shade 82°, in two minutes insolation increased to 125°, whilst 

 the temperature of shade rose 0'3 only ; on examining the sky in 

 the neighbourhood of the sun, white cirri were detected crossing its 

 disk. 



Light cloud and opalescent vapour having been thus found, when 

 in the direction of the sun, to intensify the effects of solar radiation, 

 a series of experiments was commenced with circular screens of va- 

 rious sizes, to discover, if possible, the distance to which the effects 

 of bright glare and light vapoury cloud extended round the sun. 



The observations were made when the sun's altitude was between 

 30 and 50 degrees. All the screens were placed at a uniform dis- 

 tance of six inches from the bulb of a solar thermometer | in. in 

 diameter, coated with China ink, and laid on a small piece of dark 

 oak about two inches by ten inches on grass*. The bulb of the 



* Similar results were obtained when the solar thermometer was laid upon 

 short grass, in the afternoon, when the dew was off the ground. 



With the instrument freely suspended 6 in. above the grass, the readings 

 showed a proportionate fall. 



