412 Sir James Dewar [Jan. 28, 



WEEKLY EVENING MEETING, 

 Friday, January 28, 1921. 



Sir James Crichton-Browne, J. P. M.D. LL.D. F.R.S., 



Treasurer and Vice-President, in the Chair. 



Sir James Dewar, M.A. LL.D. D.Sc. F.R.S. M.R.I., 

 Fullerian Professor of Chemistry. 



Cloudland Studies 



The measurement of sky radiation has received, attention for at 

 least a century and a half. Among the earliest investigators was 

 Leslie, who in 1818" obtained some very striking results by novel 

 applications of his differential thermometer or pyroscope. Fig. 1 is 

 a reproduction of the plate published by him. By the silvered 

 reflecting cup the " aerial pulses " he wished to measure were focussed 

 on one of the bulbs of the thermometer, while the other bulb took 

 the temperature of the surroundings. Very considerable effects were 

 thus recorded. 



The instrument so employed Leslie termed an aethrioscope, which 

 "extends its sensations through indefinite space, and reveals the 

 conditions of the remotest atmosphere." He could direct it to 

 different regions of the sky, and found the effects to remain equal 

 from the zenith down to about 20° inclination. The "cold pulses 

 shot downwards from an azure sky " sometimes produced as much as 

 5° C difference between the two bulbs of his instrument. He speaks 

 of " the tide of heat vibrated from a surface " which was " propa- 

 gated through the aerial medium by some peculiar process," although 

 he regarded this " pulsatory emission " as " auxiliary to the other 

 modes of restoring the equilibrium," adding that it "contributes a 

 very small share only towards the general effect." He also showed 

 that with the reflecting cup of his instrument directed downwards, 

 the clear sky above produced no effect until a polished silver tray 

 was held between the open reflecting cup and the ground, when a 

 negative effect of 2J° C. was produced, reduced to 0*2° C. if a sheet 

 of glass were used. If the tray was filled with water the effect was 

 entirely extinguished. The altitude of the cloud layer he regarded 

 as the important deciding factor. A range of low clouds acted as a 

 complete screen, as seen when the instrument, exposed and equili- 

 briated under a clear sky, responded instantly to the' passage of a 



Proc. Boy. Soc. Ed.., viii. 



