116 HEAT OE EARTH'S OUTER CRUST. 



the sensible effect is so instantaneous, as to leave 

 no room for supposing that the air — one of the 

 worst conductors of heat — can have the slightest 

 share in producing it. 



However the heating rays of the sun cannot 

 pass down to the surface of the earth, without their 

 power being weakened. A part always remains 

 behind in the air. This portion, which in per- 

 fectly clear air is inconsiderable, increases when the 

 atmosphere is obscured by mists, and may even 

 amount, when the heavens are overcast with thick 

 clouds, to the whole of the heat flowing in from 

 the sun. The heat thus absorbed by the clouds 

 contributes nevertheless but slightly, or not at all, 

 towards raising the temperature of the air, since it 

 is chiefly employed in re-converting these mists, 

 which consist of the precipitated moisture of the 

 air, into invisible vapour; and thus this warmth 

 passes into the state of combined heat, which is 

 imperceptible by the thermometer. 



A second loss incurred by the heat on its way to 

 the earth, depends upon the gradually increasing 

 density of the air from above downwards; for 

 the sun's rays, on passing from a rarer to a denser 

 medium, suffer a refraction, which, as we learn 

 from experiment, is always attended by a partial 

 reflection. By these continual reflections, the solar 

 rays must, in their descent through the denser and 

 denser strata of the atmosphere, be deprived of 



