1869.] Mr. J. P. Harrison on Solar Radiation. 



517 



well free again ; then rose 6° as light cloud again crossed it. The air 

 was perfectly still. 



On May 15, 1 8 6 8, 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 temperature of shade 82°, in 

 two minutes iosolation 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 various sizes, to dis- 

 cover, 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 distance 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 thermometer was not covered 

 with an exhausted globe. The mean results of the experiments were as 

 follows* : — 



1. A screen Jin. in diameter reduced the difference of the readings of 

 the black-bulb thermometer and a thermometer in the shade, four yards 

 distant, by one-third. 



2. A screen 2|ins. in diameter reduced the difference by two-thirds. 

 On reversing the experiment, converse results were obtained, e. g. 



The rays of the sun, after passing through a circular aperture 2| ins. 

 in diameter in a 12-in. screen, were made to fall on the bulb of the solar 

 thermometer, when the readings were found to equal in value those ob- 

 tained when the instrument was entirely exposedf. 



And no difference was noticed when the black-bulb thermometer was 

 screened from the rest of the sky by a double cover of mill-board placed 

 tent-wise over it. 



* 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. 



t In the above experiments, it is evident that the whole of the results were not due to 

 direct radiation or reflection. Account must be taken of the greater or less distance of 

 the heated surface of the ground, and of the hot air in contact with it, from the bidb of 

 the solar thermometer, 



