Radiant Heat, and its Conversion thereby into Sound. 523 



mitted to say that I always considered it one of his strongest 

 points — my holding of this opinion being, however, dependent 

 on the views which I entertained, and which were opposed to 

 those of Magnus, regarding the relation of liquid to vapour. 

 If, as I believe, the absorbent power is not enhanced by con- 

 densation — if in this respect water behaves like hydride of 

 aniyl and sulphuric ether, — then I do not think that such a 

 process of reverberation, between earth and clouds, as that 

 assumed by Wells is possible. The aqueous vapour, in a very 

 few thousand feet of air, of average humidity, would, if con- 

 densed, form a layer of water 0'5 of an inch in thickness; and 

 through such a layer, or even through a thinner layer, the 

 earth's radiation could not pass. If the earth's radiation 

 reach the clouds, it must be by a process similar to that of 

 handing buckets from man to man in the case of a fire. The 

 heat must be taken up and re-radiated, we know not how 

 many times, before the clouds are reached. I do not, how- 

 ever, think this mechanism of discharge necessary. Low 

 clouds will not form above exposed thermometers, in weather 

 previously serene, unless some change has occurred in the 

 atmosphere; and change may occur where no cloud reveals it. 

 It may extend, and in most cases probably does extend, from 

 the low clouds to the earth. I think it in the highest degree 

 probable that in most, if not in all, the cases cited by "Wells of 

 thermometers rising when clouds were formed overhead, the 

 precipitation was due to the intrusion of humid air, the 

 humidity extending invisibly from the clouds downwards. To 

 this, I believe, rather than to any immediate exchange of tem- 

 perature with the clouds, the rapid and considerable changes 

 of temperature referred to by him at pp. 156 and 157 of the 

 Essay are to be ascribed. Future observations will, doubt- 

 less, bring this view to an experimental test. 



I here recur with renewed pleasure to a paper published 

 by General^ Strachey in the Philosophical Magazine for July 

 1866. It was probably intended as a reply to the strictures 

 of Magnus; and to me it appears cogent in the highest degree. 

 General Strachey calculated the fall of temperature from 

 6 b 40 m p.m., Madras time, to 5 h 40 m next morning, for a cer- 

 tain number of days, selected as sufficiently clear. He also 

 calculated the mean vapour-tension during the nights, and 

 tabulated the results according to the quantity of vapour, for 

 the years 1841, 1842, 1843, and 1844. In such observations, 

 as pointed out by Strachey, discrepancies are to be expected; 

 but the general result is unmistakable, that the fall of tem- 

 perature by radiation is greatest when the air is driest, and 

 least when the air is most humid. A series of observations 



