1921] 



on Cloudland Studies 



413 



dark cloud. In general "the frigorific action was found always to 

 diminish as the clouds descend." 



Pouillet, in 1837," in conjunction with his measurement of the 

 intensity of solar energy, devised an " actinometer " for determining 



Fig. 1. 



the temperature of the zenith or upper atmosphere. This instru- 

 ment (Fig. 2) consisted simply of a thermometer resting on a bed of 

 layers of swansdown interleaved with thin discs of polished silver, a 



* Gomptes Rendus, July 4, 1838, translated in Taylor's Scientific Memoirs, 

 iv. 1896, p. 44. 



