290 CLIMATE. [SECT. xxvi. 



years will give it with tolerable accuracy ; for, although the 

 temperature in any place may be subject to very great 

 variations, yet it never deviates more than a few degrees 

 from its mean state, which consequently offers a good 

 standard of comparison. 



If climate depended solely upon the heat of the sun, all 

 places having the same latitude would have the same mean 

 annual temperature. The motion of the sun in the ecliptic, 

 indeed, occasions perpetual variations in the length of the 

 day, and in the direction of the rays with regard to the 

 earth ; yet, as the cause is periodic, the mean annual tem- 

 perature from the sun's motion alone- must be constant in 

 each parallel of latitude ; for it is evident that the accumu- 

 lation of heat in the long days of summer, which is but 

 little diminished by radiation during the short nights, is 

 balanced by the small quantity of heat received during the 

 short days in winter, and its radiation in the long, frosty, 

 and clear nights. In fact, if the globe were everywhere 

 on a level with the surface of the sea, and of uniform sub- 

 stance, so as to absorb and radiate heat equally, the mean 

 heat of the sun would be regularly distributed over its 

 surface in zones of equal annual temperature parallel to 

 the equator, from which it would decrease to each pole as 

 the square of the cosine of the latitude ; and its quantity 

 would only depend upon the altitude of the sun and 

 atmospheric currents. The distribution of heat, however, 

 in the same parallel, is very irregular in all latitudes ex- 

 cept between the tropics, where the isothermal lines, or the 

 lines passing through places of equal mean annual tempe- 

 rature, are more nearly parallel to the equator. The causes 

 of disturbance are very numerous; but such as have the 

 greatest influence, according to M. de Humboldt, to whom 

 we are indebted for the greater part of what is known on 

 the subject, are the elevation of the continents, the dis- 

 tribution of land and water over the surface of the globe 

 exposing different absorbing and radiating powers ; the vari- 



