518 



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



Results of an equally negative kind were obtained in the case of other 

 experiments which were made with the object of detecting heat in the 

 light reflected from sky and cloud not in the direction of the sun. 



A black-bulb thermometer, after having been placed for some time in a 

 dark room, was then exposed to the sky, near a large French window, facing 

 S.E., the. glass of which was clear, and had been carefully cleaned, without 

 any rise being perceptible. The sun, at an altitude of about 40°, was 

 shining brightly on white vapour and light cirro-cumuli*. 



Thermometers were also placed in the open air on the north side of the 

 house, on a still day, exposed to half the sky when covered with bright 

 white clouds ; but the mercury stood at the same height as in a dark 

 passage on the same side of the buildingt- 



The same apparent absence of any direct heating-power in the light re- 

 flected from the sky generally was shown in this, as in the previous series 

 of experiments when the solar thermometer was screened, excepting in the 

 direction of the sun. 



As respects the momentary increase of insolation which occurs in 

 connexion with bright vapour in the neighbourhood of the sun, further 

 experiment is required for the purpose of ascertaining whether it is due to 

 radiation or to reflection. 



Note. — An opportunity occurred on the 7th of June of repeating the 

 experiments with screens at altitudes of the sun exceeding 50°. The 

 following results were obtained : — 

 h m 



At 0. B. B. 110. Temp, of shade 73. rSky cloudless, but with a good deal 

 (Exposed to the sun and sky.) < of white vapour, more especially 



L about the sun. 



4. B. B. 90. Temp, of shade 73. 



(Shaded from sun by a 2-in. screen.) 

 30. B. B. 104. Temp, of shade 73. Light any 



(Exposed to sun and sky.) 



35. B. B. 94. Temp, of shade 73. Light air. 



(Shaded from sun by a -J-in. screen.) 



1 0. B. B. 108. Temp, of shade 74. Quite calm. 



(Exposed to sun and sky.) 

 1 5. B. B. 109. Temp, of shade 74. Quite calm. 



(Exposed to sun through a 2-in. circular aperture in a 12-in. screen.) 

 1 15. B. B. 108. Temp, of shade 74. Quite calm. 



(Exposed to sun and sky.) 

 1 18. B. B. 106. Temp, of shade 74. Quite calm. 



(Exposed to sun through a 2-in. circular aperture in a 12-in. screen. 

 1 20. B. B. 106. Temp, of shade 74. Quite calm. 



(Exposed to sun but screened from sky.) 



* Experiments were also tried with a 7-inch lens, without result, 

 t The thermometer exposed to the sky would probably have stood lower than the 

 one in the house if the sky had been perfectly clear. 



