Radiant Heat, and its Conversion thereby into Sound. 519 



at 100 feet above the surface, Wells invoked the radiant power 

 of the air itself. It is chilled, he thought, by its own emission 

 against the cold earth underneath. Wells takes great pains 

 to prove that the air possesses this power ; and if not the air, 

 the floating matter of the air will, he contends, exert the 

 necessary radiation. Difficulties of this nature not unfre- 

 quently crop up in works on meteorology ; but they disappear 

 in presence of the fact that mixed with the air is a gaseous 

 constituent, small in quantity, but capable of producing the 

 effects needing explanation. 



As an example of such difficulties, I have already referred 

 to Sir John Leslie's paper, " On certain Impressions of Cold 

 transmitted from the Higher Atmosphere " *. He there de- 

 scribes the iEthrioscope, an instrument used to measure these 

 impressions. " The sensibility," he says, " of the instrument 

 is very striking ; for the liquor incessantly falls and rises in 

 the stem with every passing cloud. Under a fine blue sky, 

 it will sometimes indicate a cold of 50 millesimal degrees ; 

 yet on the other days, when the air seems equally bright, 

 the effect is only 30°. The causes of these variations are 

 not quite ascertained." He might have said, not at all 

 ascertained. The causes, I submit, are the variations of the 

 quantify of transparent aqueous vapour in the atmosphere, 

 which, without affecting the visual brightness of the air, is 

 competent to arrest radiation from the earth. Precisely of 

 the same character is the difficulty noticed by Lieutenant 

 Hennessey in his paper on " Actinometrical Observations in 

 India/' Like Leslie, he speaks of variations the causes of 

 which are not ascertained. "Again," he says, "there is a 

 change of intensity from day to day apparently not due to 

 alterations in the sun's declination so that the average daily 

 curve (about noon) is higher or lower without any visible 

 reason "f . The reason here is that applicable in Leslie's case, 

 namely the variations of the invisible atmospheric vapour. 



In 1866 my friend Professor Soret, of Geneva, favoured me 

 with a letter from which the following is an extract : — " In 

 two comparative experiments, made within a few days at 

 Geneva and Bologna, the most powerful radiation was ob- 

 tained at Geneva, although at Bologna the heavens were 

 visibly purer. The result appears to me to support your views 

 regarding the aqueous vapour of the air ; for the tension of 

 aqueous vapour at Bologna was 10*7, while at Geneva it was 

 only 6-33." 



Cautiously abstaining from drawing a general conclusion 



* Transactions Roy. Soc. of Edinburgh, vol. viii. p. 483. 

 t Proc. Roy. Soc. vol. xix. p. 228. 

 2Q2 



