J. Croll—Geological Climatology. 261 
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 sup- 
poses 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 conserva- 
tion of snow, then there is no ground whatever for any embar- 
rassment about the matter. 
“We 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 misap- 
prehensions is the somewhat complex nature of the subject 
arising from the fact that aqueous vapor acts so very differ- 
ently under different conditions. When the vapor exists 
Mm 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 
Tays 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 Jower the general temperature. But when this vapor 
condenses into thick fogs it acts in a totally different manner. 
The transparency to a great extent disappears, and the fog then 
Cuts off the sun’s rays and prevents them from reaching the 
ground. This it does in two different ways. Ist. Its watery 
particles, like the crystals of the snow, are good reflectors, and 
the upper surface of the mass of fog on which the rays fall 
acts as a reflector, throwing back a large portion of the rays 
Into stellar space. The rest of the rays which are not reflected 
enter the fog and the larger portion of them are absorbed by 
it. But it will be observed that by far the greater part of the 
absorption, if not nearly all of it, will take place in the upper 
half of the mass. This is a necessary result of a recogn 
principle in radiant heat known as the “sifting” of the rays. 
he deeper the rays penetrate into the fog the less will be the 
amount of heat absorbed. If the depth of the mass be great, 
