Radiant Heat, and its Conversion thereby into Sound. 515 

 Time of observation. 



8* 



P.M. 



9 



JJ 



10 



55 



11 



JJ 



Hi 



55 



Snow-temperature. 



Air 



-temperature. 



o 





o 



-12 





-0 



-14 





-2 



-14 





-4 



-17 





-6 



-18 





-6 



-20 





-8 



-23 





-7 



The sign — signifies that the temperatures were all below ; 

 zero Fahrenheit. These temperatures amply justify Wilson's 

 statement that the cold was " extraordinary." Coexistent 

 moreover with the general cold, we have a difference of 16° 

 between the temperature of the surface and that of the air 

 2 feet above it. Had the air-thermometer been hung 10 feet 

 instead of 2 feet above the surface, the difference would have 

 been still greater. The thermometer, moreover, must have 

 been chilled, not only by its immersion in cold air, but also 

 by its own radiation against the intensely cold snow. The 

 chilling of the superficial snow was purely an effect of radia- 

 tion. Beneath the surface its temperature reached +14°. 

 Wilson filled a bread-basket with this warm snow at 2-g- A.M. 

 on the 14th. Within half an hour it had fallen 24°, and in 

 two hours 32°. 



I venture to predict that if Wilson's experiment be repeated 

 during the cold of a Canadian winter the same result will be 

 obtained ; and it seems to me that until the action of water- 

 vapour upon radiant heat had been discovered, no explanation 

 of the phenomenon could have been given. It was accepted, 

 but not accounted for. On the night of Wilson's observations 

 " a light air was felt coming from the east." With such an 

 "air" and such a temperature the quantity of water- vapour 

 in the atmosphere must have been infinitesimal. Dry air 

 being a practical vacuum to the rays of heat, were the vapour- 

 screen entirely removed, the earth would find itself exchanging 

 temperatures with celestial space, and the superficial chill 

 would be commensurate. In Wilson's case, though vapour 

 was not abolished, it was so far diminished as to produce the 

 observed refrigeration. Meteorologists, I am informed, some- 

 times say that laboratory experiments, however well performed, 

 have but little application to their field of observation*. I, 



* Mr. Hill, the Meteorological reporter for the North-western Provinces 

 of India, writes thus : — " There is even, 021 the part of some, an evident 

 reluctance to accept the decision of laboratory experiments on the ques- 

 tion of atmospheric absorption as final, however ingeuious, varied, and 

 consistent with one another the experiments may be " (Proc. Roy. Soc. 

 vol. xxxiii. p. 216). 



