RADIANT HEAT, AND ITS CONVERSION THEREBY INTO SOUND. 351 
Magnus urged this point against me, and I may be permitted 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 condensation—if in this respect water behaves like hydride 
of amyl 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 condensed, 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, however, think this 
mechanism of discharge necessary. Low clouds will not form above exposed thermo¬ 
meters, in weather previously serene, unless some change has occurred in the atmos¬ 
phere ; 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 ther¬ 
mometers 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 temperature 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, doubtless, 
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 w T as 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 h 40 m p.m., Madras time, to 5 h 40 m 
next morning, for a certain number of days, selected as sufficiently clear. He also 
calculated the mean vapour tension during the nights, and tabulated the results accord¬ 
ing 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 temperature by radiation is greatest 
when the air is driest, and least when the air is most humid. A series of observations 
made at Madras between the 4th and the 25th of March, 1850, are particularly suit¬ 
able for the illustration of this law of action. During the period referred to “the sky 
remained remarkably clear, while great variations of the quantity of vapour took 
place.” Here are the results as tabulated by General Strachey :— 
have sometimes seen on grass, though the sky was entirely hidden, no very inconsiderable quantity of 
dew.”—Ibid., p. 128. 
