SECT, xxvi.] ISOGEOTHERMAL LINES. 289 



Since the warmth of the superficial strata of the earth 

 decreases from the equator to the poles, there are many 

 places in both hemispheres where the ground has the same 

 mean temperature. If lines were drawn through all those 

 points in the upper strata of the glohe which have the 

 same mean annual temperature, they would be nearly 

 parallel to the equator between the tropics, and would 

 become more and more irregular and sinuous towards the 

 poles. These are called isogeothermal lines. A variety of 

 local circumstances disturb their parallelism, even between 

 the tropics. 



The temperature of the ground at the equator is lower on 

 the coasts and islands than in the interior of continents ; 

 the warmest part is in the interior of Africa ; but it is ob- 

 viously affected by the nature of the soil, especially if it be 

 volcanic. 



Much has been done within a few years to ascertain the 

 manner in which heat is distributed over the surface of our 

 planet, and the variations of climate, which, in a general 

 view, mean every change of the atmosphere, such as of tem- 

 perature, humidity, variations of barometric pressure, purity 

 of air, 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 ab- 

 sorbing and emitting or radiating heat. When the inter- 

 change is equal, the temperature of a body remains the same ; 

 but, when the radiation exceeds the absorption, it becomes 

 colder, and vice versd. In order to determine the distribution 

 of heat over the surface of the earth, it is necessary to find a 

 standard by which the temperature 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 through- 

 out the earth. The annual average temperature may be found 

 by adding the mean temperatures of all the months in the year, 

 and dividing the sum by twelve. The average of ten or fifteen 



