292 OUTLINES OF NATURAL PHILOSOPHY. 



sons of the year. In consequence of these changes, 

 the quantities of light and heat received by the earth 

 are not proportional to the times, but to the angles 

 described by the earth round the sun in those times. 

 These variations, however, are but inconsiderable, 

 and as they are annual, they do not produce any ine- 

 quality in the whole heat or light of one year compa- 

 red with those of another. 



394. Though the earth is thus receiving heat 

 continually, and nearly at the same rate, its ave- 

 rage temperature appears to remain invariable ; as 

 much heat as comes from the solar rays, flying 

 off constantly into the space, whether empty or 

 occupied by subtle matter, which surrounds the 

 earth. 



395. While the general temperature of the earth 

 remains invariable, the distribution of heat over 

 its surface is extremely unequal, being different in 

 different places, and in the same place subject to 

 variations, both regular and irregular. 



396. The causes which determine the distribu- 

 tion of heat over the earth's surface, are either the 

 direct influence of the solar rays, or the communi- 

 cation of heat by the air from one part of the earth's 

 surface to another. 



397. The 



