SCT. XXVI. CLIMATE. 253 



A variety of local circumstances disturb their parallelism 

 even between the tropics. 



The temperature of the ground at the equator is 

 Jower on the coasts and islands than hi the interior of 

 continents ; the warmest part is in the ulterior of Africa, 

 but it is obviously affected by the nature of the soil, es- 

 pecially if it be volcanic. 



Much has been done within a few years to ascertain 

 the manner in which heat is distributed over the sur- 

 face of our planet, and the variations of climate, which 

 in a general view mean every change of the atmos- 

 phere, such as of temperature, humidity, variations ot 

 barometric pressure, purity of ah*, the serenity of the 

 heavens, the effects of winds, and electric tension. 

 Temperature depends upon the property which all 

 bodies possess more or less, of perpetually absorbing and 

 emitting or radiating heat. When the interchange is 

 equal, the temperature of a body remains the same ; 

 but when the radiation exceeds the absorption, it be- 

 comes colder, and vice versa. In order to determine 

 the distribution of heat over the surface of the earth, it 

 is necessary to find a standard by which the tempera- 

 ture in different latitudes may be compared. For that 

 purpose it is requisite to ascertain by experiment the 

 mean temperature of the day, of the month, and of the 

 year, at as many places as possible throughout the 

 earth. The annual average temperature may be found 

 by adding the mean temperatures of all the months hi 

 the year, and dividing the sum by twelve. The average 

 of ten or fifteen years will give it with tolerable accu- 

 racy ; 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 temperature from the sun's motion alone 

 must be constant in each parallel of latitude. For it is 

 Y 



