in Geological Climatology. 253 
sky would effectually prevent the sun's rays from reaching the 
earth, and the suotv in consequence would remain unmelted 
during the entire summer/ 
On this Professor Xewcornb's criticism is as follows : — 
" Here he Qlr. Croll) says nothing about the latent heat set 
free by the condensation, nor does he say where the heat 
goes to which the air must lose in order to be chilled. The 
task of arguing with a disputant who in one breath maintains 
that the transparency of the air is such that the rays reflected 
from the snow pass freely into space, and in the next breath 
that thick fogs effectually prevent the rays ever reaching the 
snow at all, is not free from embarrassment.'"' 
If he really supposes my meaning to be that the air is so 
transparent as to allow the incident and reflected rays of the 
sun to pass freely without interruption, while at the same 
time and in the same place the air is not transparent but filled 
with dense fogs which effectually cut off the sun's rays and 
prevent them from reaching the earth, then I do not wonder 
that he should feel embarrassed in arguing with me. But if 
he supposes my meaning to be. as it of course is, that those 
two opposite conditions, existing at totally different times or 
in totally different places at the same time, should lead to 
similar results, namely the cooling of the air and consequent 
conservation of snow, then there is no ground whatever for 
any embarrassment about the matter. 
•• TTe might therefore show/' he states, " that if the snow, 
air, fog, or whatever throws back the rays of the sun into 
space is so excellent a reflector of heat, it is a correspondingly 
poor radiator : and the same fog which will not be dissipated 
by the summer heat will not be affected by the winter's cold, 
and will therefore serve as a screen to prevent the radiation 
of heat from the earth during the winter/' 
There are few points in connection with terrestrial physics 
which appear to be so much misunderstood as that of the 
influence of fogs on climate. One chief cause of these mis- 
apprehensions is the somewhat complex nature of the subject 
arising from the fact that aqueous vapour acts so very differ- 
ently under different conditions. When the vapour exists in 
the air as an invisible gas, we have often an intensely clear and 
transparent sky, allowing the sun's rays to pass to the ground 
with little or no interruption ; and if the surface of the 
ground be covered with snow, a large portion of the incident 
rays are reflected back into space without heating either the 
snow or the air. The general effect of this loss of heat is. of 
course, to lower the general temperature. But when this 
vapour condenses into thick fogs it acts in a totally different 
