44 DISCUSSIONS IN CLIMATOLOGY. 



appears, 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 absorp- 

 tion, if not nearly all of it, will take place in the upper 

 half of the mass. This is a necessary result of a recog- 

 nised 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 pro- 

 bably 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 prin- 

 cipal 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 cer- 

 tainly 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 deprivation of sun-heat of a much less 

 extent than this would certainly lower the summer 



