254 Dr. J. Croll on some Controverted Points 
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. 
1st. 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 recognized principle in radiant heat 
known as the " sifting " of the rays. The deeper the rays 
penetrate into the fog, the less will be the amount of heat 
absorbed. If the depth of the mass be great, absorption will 
probably entirely disappear before the surface of the ground 
is reached. The fog will begin, of course, to radiate off the 
heat thus absorbed ; but as it is the upper half of the mass 
which has received the principal part of the heat, the most of 
this heat will be radiated upward into stellar space, and, like 
the reflected heat, entirely lost in so far as heating the earth 
is concerned. A portion will also be radiated downward, 
some of which may reach the ground, but the greater portion 
will be reabsorbed in its passage through the mass. We have 
no means of estimating the amount of heat which would thus 
be thrown off into space by reflection and radiation ; but it 
is certainly great. I think we may safely conclude that in 
places like South Georgia and Sandwich Land, where fogs 
prevail to such an extent during summer, one half at least of 
the heat from the sun never reaches the ground. A depriva- 
tion of sun-heat of a much less extent than this would 
certainly lower the summer temperature of these places far 
below the freezing-point, were it not for a compensating cause 
to which I shall now refer, viz. the heat " trapped " by the 
fog. The fog, although it prevents a large portion of the 
sun's heat from ever reaching a place, at the same time 
prevents to a great extent that place from losing the little 
heat which it does receive. In other words, it acts as a 
screen preventing the loss of heat by radiation into space. 
But the heat thus " trapped " never fully compensates for 
that not received, and a lowering of temperature is always 
the result. 
Had all those considerations been taken into account by 
Professor Newcomb, Mr. Hill, Mr. Searles Wood, and others, 
they would have seen that I had by no means overestimated 
the powerful influence of fogs in lowering the summer 
temperature. 
