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 imal 
portion penetrates into the soil, but the greatest share is dispersed 
by communication to the ambient air. In the progress of the 
302 } day, 
