1921] on Cloudland Studies 415 



the observed lowering of temperature of the actinometer thermometer 

 with the known temperature of the cold dome. This empirical 

 expression was then applied to the readings obtained by exposure to 

 the night sky, and the corresponding zenith temperature obtained. 

 His empirical result was that the observed lowering of temperature 

 of the actinometer thermometer below that of the adjacent earth 

 was 4/9 of the actual amount of the " zenithal enclosure " below the 

 earth temperature — i.e. 



[—-5'] 



where Z - temperature of zenith (or cooled dome) ; t = temperature 

 of earth ; and d = observed lowering of temperature of the actino- 

 meter. Some of Pouillet's results are given in Table I. 



Melloni, well known in connection with measurements of heat 

 transmissivity, compared the readings of two thermometers exposed 

 to the night sky, one of the thermometer bulbs being blackened. 

 He found that the blackened thermometer under clear skies was 

 cooled relatively to the plain one by an amount which was almost 

 independent of the temperature of the air: This surprising result 

 has been discussed and receives support in the course of an extended 

 study of the probjem of atmospheric and nocturnal radiation carried 

 out by Anders Angstrom and many others associated with the 

 Smithsonian Institution in measurements of the distribution of solar 

 energy.* These modern methods are based on the use of various 

 forms of electrical thermometers or pyrometers, in which thin 

 blackened strips of metal, forming part of a measuring circuit, are 

 cooled or heated by exposure to the sky or sun, as the case may be, 

 and the alteration in temperature compensated by electric energy 

 whose value is very accurately determined. The absolute value of 

 the radiation exchange of a strip of known dimensions is then deduced 

 by applying the Stefan " fourth power " radiation law. 



Measurements of the solar radiation were made in this way at 

 Washington in 1902, and were repeated later at Mount Wilson, at 

 Calama in Chile, and at Harqua Hala in Arizona. Observations 

 were made by Abbott and Aldrich on Mount Whitney, and by 

 Angstrom with the Smithsonian expedition to Bassour in Algeria ; 

 and continuous observations are now carried on in several favourable 

 localities on the American continent. 



The instruments used, known as pyranometers, pyrgeometers, etc., 

 can be calibrated by exposure in a quiet enclosure at known tempera- 

 tures. When in use they are fully exposed to the sky without any 

 shelter, and occasionally show a secondary effect from the cooling or 

 warming of their metal supports and fittings, some forms being 

 more affected than others. It would be interesting if comparative 



* Smithsonian Coll., 65, 1916. 



