RADIANT HEAT, AND ITS CONVERSION THEREBY INTO SOUND. 
345 
peratures with celestial space, and the superficial chill would be commensurate. In 
Wilson’s case, though the vapour was not abolished, it was so far diminished as to 
produce the observed refrigeration. Meteorologists, I am informed, sometimes say that 
laboratory experiments, however well performed, have but little application to their 
field of observation.'* * * § I, on the other hand, submit that such experiments are necessary 
to rescue their science from empiricism. What could Wells have done with dew had 
he not been preceded by Leslie and Rumfobd ? His whole theory is an application 
of results obtained in the laboratory.! 
What I have stated regarding Wilson applies also to Six, who concluded from his 
experiments “ that the greatest differences at night in point of temperature, between 
bodies on the surface of the earth and the atmosphere near it, are those which take 
place in very cold weather.” This is quoted from Wells,}; who, in his Essay on Dew, 
recurs more than once to the subject. He signalises, but does not explain, “the 
greater difference which takes place in very cold weather, if it be calm and clear, 
between the temperatures of the air and of bodies on the earth at night, than in 
equally calm and clear weather in summer.”§ A considerable number of observations 
bearing upon this point are scattered through the Essay. The radiant power of the air 
being practically nil, it retains for a considerable time the warmth imparted to it 
during the day, while when it is dry, the rays from the surface of the earth pass 
unimpeded through it. Hence the relative refrigeration of the surface.! 
In regard to the action of water vapour Magnus considered experiment superfluous, 
as the phenomenon of dew sufficed to prove me wrong. If the vapour possessed the 
power which I ascribed to it, he contended that dew could not be formed. It is not 
difficult to dispose of this objection. The formation of dew and superficial refrigeration 
are connected, not by coincidence but by opposition. I would venture to predict that 
where the one is great the other, in general, will be small. “'Very little dew,” says 
* Mr. Hill, the Meteorological Reporter for the North-Western Provinces of India, writes thus:— 
“ There is even, on the part of some, an evident reluctance to accept the decision of laboratory experi¬ 
ments on the question of atmospheric absorption as final, however ingenious, varied, and consistent with 
one another the experiments may be.”—Proc. Roy. Soc., vol. 33, p. 216. 
f “Its complete theory,” says Wells, “ could not possibly, in my opinion, have been attained, before 
the discoveries on heat were made, which are contained in the works of Mr. Leslie and Count Rumford.” 
—Essays, p. 191. 
t Essays, p. 176. Wells thus generously refers to the labours of Wilson. “Indeed, several of my 
experiments upon dew were only imitations of some which had been previously made on hoar frost, by 
that ingenious and worthy man.”—Essays, p. 151. 
§ Ibid., p. 188. 
|| It ought to be stated that, contrary to Six and to Wells, Mr. Glaisher has found that “the 
differences between the temperature of the air and of bodies on the earth at night, in equally clear and 
calm weather, were the same at every period of the year.” (Phil. Trans., 1847, p. 126.) He moreover 
records differences considerably in excess of those observed by Wilson and by Wells. Keeping the 
action of aqueous vapour in view, the elaborate paper of Mr. Glaisher might repay further discussion, 
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