SUBDIVISIONS OF GEOGRAPHY. 



o. The subdivisions of physical geography are sometimes 

 denominated HYDROLOGY, which includes all that relates to the 

 ocean and seas; METEOROLOGY, which includes the investigation 

 of atmospheric phenomena ; and CLIMATOLOGY, which involves 

 the consideration of the mean temperature of different countries, 

 the altitude of the line above which there is perpetual snow, the 

 prevailing winds, the barometric pressure, the annual quantity of 

 rain, and so on. 



6. The earth, considered as the abode of mankind, distributed 

 into different nations and placed under different forms of govern- 

 ment, constitutes the subject of POLITICAL GEOGRAPHY, which 

 therefore includes the consideration of the moral and social con- 

 dition of different peoples, their language, religion, forms of 

 government, and civilisation, and the population, resources and 

 local relations of different countries. 



7. It appears, therefore, that these several divisions of geo- 

 graphical science are limited to the appearances and phenomena 

 developed upon the surface of the earth, in the waters which par- 

 tially cover it, and in the atmosphere by which it is surrounded. 



8. Another, and not less important department of science con- 

 cerning the earth, relates to the condition and structure of those 

 parts of the globe which lie between its surface and its centre, and 

 which constitute the subject of GEOLOGY. 



9. In mathematical geography and astronomy, certain physical 

 circumstances attending the interior of the earth have been 

 developed. Thus it is proved that the density of the globe 

 gradually increases from the surface to the centre ; and from its 

 peculiar form it is inferred that, at the epoch of its formation, 

 the materials composing it must have been in a fluid state. It 

 has been already shown in our Tract on the "Earth," that the 

 form of the globe is what in geometry is called an oblate spheroid, 

 a figure somewhat resembling an orange or a turnip. By reason 

 of this figure, the earth is flattened at the poles and bulged out at 

 the equator. Now it is proved in mathematical physics that if a 

 fluid globe have a motion of rotation on one of its diameters as 

 an axis, the centrifugal force of the matter composing it will cause 

 it to bulge out at the equator and to flatten at the poles, so that all 

 sections of it made by planes passing through the axis are ellipses, 

 and the degree of the elliptic form of these sections will be so 

 much the greater as the velocity of rotation is more rapid ; and so 

 closely connected is this degree of ellipticity with the velocity of 

 rotation, that mathematicians are able to assign the exact form 

 of the elliptic section which must correspond to each particular 

 velocity of rotation. 



10. Before the exact form of the earth had been determined by 



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