418 Proceedings of the Royal Society 
weather, annihilates the difference; and that such is also the effect 
of the atmosphere becoming hazy, or the sky overcast with clouds. 
Wilson laboured to account for all these variable phenomena on the 
assumption that the deposition of hoar-frost occasioned the cold. 
But it is easy to see that, in spite of some ingenious suggestions 
upon that basis, he did not succeed in satisfying entirely his own 
mind. Had he begun with the converse assumption, that cold was 
the cause of deposition of hoar-frost, he would probably have antici- 
pated more modern discovery. He was indeed very near doing so. 
For, speaking of the influence of a passing cloud in putting an end 
to the formation of hoar-frost and depression of the therm ometer, 
he uses these words, “ When the atmosphere becomes suddenly 
clouded, it is certain that this change must be attended with the 
extrication of much sensible heat in the higher regions, wheie 
these vapours are congregated. A store of heat so produced must 
soon affect the mass of air which lies below.” But how? he might 
have asked himself. Not surely by the process of conduction, be- 
cause heated air rises; it does not sink. Not, then, by the process 
of conduction, but by that of radiation, which, instantly darting 
heat from the clouds, replaces the loss which in clear weather is 
sustained by snow and other objects on the surface of the earth 
through radiation of heat from them into the cold attenuated atmo- 
sphere of the far firmament. Unfortunately, however, the theory 
of radiant heat was too little advanced to suggest to him this ex- 
planation, — obvious and easy after the discoveries of Pictet in 
1790, and the admirable researches of Leslie fourteen years later. 
No one could investigate carefully the theory of the winds with- 
out having his attention directed to the Theory of Bain ; and, ac- 
cordingly this is one of the branches of meteorology which Hutton 
was the first to investigate successfully. This he did in a disserta- 
tion read in 1784 and enforced in 1787, in reply to adverse criti- 
cisms by the philosopher De Luc. The power which the atmosphere 
possesses of dissolving or suspending moisture in invisible vapour 
as transparent as the air increases with the temperature. Hutton’s 
discovery was the proof, by mathematical demonstration, that rain 
or mist could not be formed when two masses of air of different 
temperatures are mingled, unless the power to dissolve moisture 
