1921] 



on Cloudland Studies 



419 



sky on this basis is shown in the sixth column. Fig. 4 shows the 

 plotted relation already referred to between water vapour and 

 nocturnal radiation. For this curve the radiation results were all 

 reduced to an earth temperature of 20° C. to get a steady basis of 

 comparison. 



Fig. 4. 



Vapour pressures mm. of Hg. 



Sky Radiation Measurements by Charcoal 

 Thermoscope. 



As the charcoal thermoscope described a year ago* has proved a 

 simple and workable instrument for comparison of radiation inten- 

 sities, it was adapted to the study of the varying radiation from the 

 sky by both day and night, and has yielded some suggestive results. 

 The sensitive saturated charcoal surface exposed, being granular in 

 texture, is quite a good absorber. The charcoal grains, graded to 

 approximately h mm., are the characteristic angular fragments 



* Proc.'Roy. Inst., xxiii., p. 245, et seq. 



