1883.] 



On Terrestrial Radiation. 



25 



need not' here be dwelt upon, my object being to illustrate a principle 

 rather than to add to the multitudinous records of meteorology. It will 

 be sufficient to say, that with atmospheric conditions sensibly alike, 

 the waste of heat from the earth varies from day to day ; a result due 

 to the action of a body which escapes the sense of vision. It is hardly 

 necessary for me to repeat here my references to the observations 

 of Leslie, Hennessey, and others, which revealed variations in the 

 earth's emission for which the observers could not account. A close 

 inspection of the observations of the late Principal Forbes on the 

 Faulhorn, proves, I think, that the action of aqueous vapour came 

 there into play, and his detection of this action, while unacquainted 

 with its cause, is in my opinion a cogent proof of the accuracy of his 

 work as a meteorologist. 



Postscript. 



In the " Philosophical Transactions " for 1882, Part I, p. 348, 1 refer 

 to certain experiments executed by Professor Soret of Geneva. My 

 friend has recently drawn my attention to a communication made by 

 him to the French Association for the Advancement of Science, in 

 1872. It gives me great pleasure to cite here the conclusions at which 

 he has arrived. 



" The influence of humidity is shown by the whole of the observa- 

 tions ; and it may be stated generally, that, other circumstances being 

 equal, the greater the tension of aqueous vapour, the less intense is 

 the radiation. 



" In winter, when the air is drier, the radiation is much more 

 intense than in summer, for the same height of the sun above the 

 horizon. 



" On several occasions a more intense radiation has been observed 

 in dry than in humid weather, although the atmosphere was incon- 

 testably purer and more transparent in the second case than in the 

 first. 



" The maximum intensity of radiation, particularly in the summer, 

 corresponds habitually to days exceptionally cold and dry." 



Such are the results of experiments, executed by a most excellent 

 observer, on the radiation of the sun. They apply word for word to 

 terrestrial radiation. They are, moreover, in complete harmony with 

 the results, published by General Strachey in the " Philosophical 

 Magazine" for 1866. Meteorologists will not, I trust, be offended 

 with me if I say that from such outsiders, fresh to the work and 

 equipped with the necessary physical knowledge, they may expect 

 efficient aid towards introducing order and causality among their 

 valuable observations. 



