FROM THE HIGHER ATMOSPHERE. 475 



gain heat with the same facility, in a small closet, as in a spa- 

 cious room. Let a glass ball of 3 inches in diameter, and 

 filled with hot water, or a frigorific mixture, be suspended in 

 the centre of a balloon of 18 inches in diameter, and it 

 will be found to change its temperature almost at the same rate 

 as in a close apartment. If the balloon were much smaller, 

 however, the effect would become sensibly diminished, because 

 the whole of the included air, being rendered somewhat hot- 

 ter or colder than before, would excite a feebler action at the 

 surface of the ball. A similar modification would take place if 

 the ball were formed of hollow silver. Yet this large shell of 

 glass evidently forms a complete screen, which will absorb all 

 the impressions emitted within it. 



The sun is the great fountain of that heat which vivifies our 

 planet. Of the solar beams, part are detained in their passage 

 through the atmosphere, and the rest are absorbed at the sur- 

 face of the earth. From experiments with the photometer, it 

 follows, that, even when the sky is most serene, only one-half 

 of the sun's light, sloping at an angle of 25°, will reach the 

 ground ; and that, at an angle of 15°, the proportion is redu- 

 ced to one-third ; but with an obliquity of 5°, only the twen- 

 tieth part of the whole can gain the surface. The effect of this 

 absorption during the day, is to raise the temperature of the 

 external crust of the globe. Very few of the incident rays are 

 again reflected from the surface. An expanse of sand, or even 

 of chalk, however offensive by its white glare, yet scarcely 

 sends back a fifth part of the light into the mass of air. A green 

 sward, or a dark soil, absorbs almost the whole of the lumi- 

 nous particles. 



Of the heat thus accumulating at the earth's surface, a small 

 portion penetrates into the soil, but the greatest share is dispersed 

 by communication to the ambient air. In the progress of the 



3 2 day, 



