Radiant Heat, and its Conversion thereby into Sound. 517 



the earth, relatively to the temperature of the air, both of 

 them having occurred after a long tract of dry weather"*. 

 This evidence is specially valuable in view of the fact that 

 Wells knew nothing of the action of water-vapour on radiant 

 heat. On another occasion he observed a difference of 9£ 

 degrees between the temperature of the air and that of wool 

 placed upon the earth, without any deposition of dew whatever 

 upon the chilled wool f. He supplements these observations 

 by one equally important in the opposite direction. " On the 

 night/' he writes, "which afforded the most copious dew ever 

 observed by me, the cold possessed by the grass beyond that 

 of the air was for the most part only 3° and 4° "\. The 

 smallness of the refrigeration in this instance, and the copious- 

 ness of the dew, I refer to one and the same cause, namely 

 the abundance of vapour. Heavy dew implies this abundance ; 

 abundant vapour, if not too local, implies checked radiation; 

 and checked radiation tends to abolish the difference of tem- 

 perature between air and soil. 



Wells had a theory of his own to account for the association 

 of moderate refrigeration and heavy dew. The heat rendered 

 free by the condensation of the vapour to liquid "prevented" 

 the cold. He tried to determine the effect of condensation 

 by the following experiment §. To 10 grains of wool he 

 added 21 grains of water, this being the quantity of dew de- 

 posited on wool in one of his observations. He placed the 

 moistened wool in a saucer on a feather-bed in a room, and 

 determined the chill produced by its evaporation. After 

 eight hours, while the wool still retained 2^ grains of mois- 

 ture, its temperature was 4° lower than that of a dry saucer 

 placed near it on the same feather-bed. When the process is 

 reversed, condensation instead of evaporation coming s into play, 

 the foregoing amount of heat, Wells contended, would be libe- 

 rated on the grass, and thus prevent inordinate refrigeration. 

 In thus reasoning Wells went to the limit of the knowledge 

 of his time; and the explanation here given is a philosophical 

 one. But I do not think it a sufficient explanation. The 

 grass is exposed to the open atmosphere ; and the heat deve- 



* Essays, p. 188. 



t Ibid. p. 183. Wells sometimes found wetted wool to lose weight, 

 while dry wool gained no weight though lowered mauy degrees below 

 the temperature of the air (p. 184). 



X Ibid. p. 169. . From a remark occurring at page 135, it may be in- 

 ferred that the night here referred to was that common to the 29th and 

 30th of July, 1813. On the two occasions first mentioned, when there 

 was but little dew, the grass was in one instance 12°, and in the other 

 14° colder than the air. 



§ Essays, p. 187. 

 Phil. Mag. S. 5. No. 84. Suppl. Vol. 13. 2 Q 



