408 



VARIATIONS OF MEAN TEMPERATURE. 



Isothermal 

 zones. 



CHAPTER pliical equatorial line passes through the continents of 

 ^ ^^ ^^' Africa and South America, leaving nearly a third of the 



Kquatnrial former, and fully five-sixths of the latter to the south of 

 the equator, while no portion of the entire Asiatic conti- 

 nent extends so far south as to the equator. The equa- 

 torial regions of Africa and South America are accordingly 

 exposed to the direct rays of the sun, influenced only by 

 such modifications as the physical conformations of either 

 continent may effect, while tiie same region, lying in the 

 Indian seas, is tempered by the meliorating and more 

 equable temperature of the ocean, so that the vast re- 

 gions of Australia which lie witliin the same parallels of 

 latitude as the southern piirt of the African continent, are 

 now supplying to the British emigrant a temperate cli- 

 mate, more constant, but little more oppressive than the 

 mild summers of the British islands. Tlie isothermal 

 zones, which are thus made to take the place for certain 

 purposes, of the older geographical divisions of the globe, 

 must be ascertained by observations entirely different 

 from those of the mathematician and the astronomer. 

 The thermometer is the chief instrument of the isother- 

 mal observer, and by means of it the globe has been 

 divided into zones of equal annual mean temperature, 

 the perfect knowledge of which involves many imjjortant 

 practical results. The inflexions of these isothermal 

 lines is often very remarkable. On the South American 

 continent, for examide, the equator of tcmpeiature de- 

 viates entirely from the geograpliical equator, being in- 

 flected entirely to the nortli, and exceeding, by a mean 

 temperature of ten degrees, the true geogi'Hphical equa- 

 tor. In Africa also it lies to the noith of the geogra- 

 phical equator, tliough the isothermal inflexion is not to 

 the same extent ; wiiile in Asia the two equatorial lines 

 almost exactly coincide, tlie island of Borneo, and the 

 Indian seas, lyini,' directly on the line of the geographical 

 equator, and within the zone of highest temperature. 

 The deviations and inflections of the isothermal zones 

 are much less mai-ked where they occur within a large 



