414 



Sir James Dewar 



[Jan. 21 



heat-protective layer being thus formed between the thermometer 

 bulb and the ground. There was, in addition, a polished silver rim 

 as an aid to obtaining quiet conditions immediately round the 

 thermometer bulb, this rim being pierced with several small holes to 

 ensure a quiet circulation of air. 



With this actinometer he endeavoured also to deduce the tem- 

 perature of space, or the amount of the planetary heat falling on the 

 earth. His results in this case are untenable, and indeed were based 



Fig. 2. 



on unsafe extensions of the laws of cooling as deduced by Dulong 

 and Petit for limited conditions, and also upon the absorptive 

 properties of diathermanous envelopes, which were largely illusory. 

 Ou the other hand, his value for the solar constant is much closer to 

 the latest accepted value than the figures obtained by several experi- 

 menters since his time. His values for the equivalent zenith tem- 

 perature were obtained by a careful calibration of his actinometer 

 exposed to a very large superimposed cold dome at known temperature. 

 These measurements gave him an empirical expression connecting 



