16 



PHYSIOGRAPHIC GEOLOGY. 



The density of the moon is 3-1, or about that of basalt; of Mercury, 6-2 ; of Venus 

 and Mars, each, 5-2 ; of Jupiter, 1-3. 



The earth's atmosphere, if considered a part of the sphere, adds several 

 hundred miles to its diameter. Its actual limit is not ascertained ; but evi- 

 dence from meteorites places it at least 200 miles above the earth's surface. 



(2) General subdivisions of the earth's surface. — Proportion of land and 

 loater. — In the surface of the sphere there are about 73 % of water to 27 % 

 of dry land. The proportion of land north of the equator is nearly three 

 times as great as that south. The zone containing the largest proportion 

 of land is the north temperate, the area equaling that of the water ; while 

 it is only one third that of the water in the torrid zone, and hardly one 

 tenth (gx) in the south temperate. 



Out of the 196,900,000 of square miles which make up the entire 

 surface of the globe, 144,155,000 are water and 52,745,000 land. In the 

 northern hemisphere the land covers 38,780,000 square miles, and the water 

 59,670,000; in the southern, the land 13,965,000 square miles, the water 

 84,485,000. 



Land in one hemisphere. — If a globe be cut through the center by a 

 plane intersecting the meridian of 175° E. at the parallel of 40° N., one of 

 the hemispheres thus made, the northern, will contain nearly all the land of 

 the globe, and the other be almost wholly water. The annexed map repre- 

 sents the two hemispheres. 



1.^ o if> 



15" — tr~^^ 



The pole of the land-hemisphere in this map is in the western half of 

 the British Channel ; and, if this part, on a common globe, be placed in the 

 zenith, under the brass meridian, the horizon-circle will then mark the line 

 of division between the two hemispheres. Of the 98,450,000 square miles 

 of surface in each hemisphere, there are about 45,000,000 of land in the 

 land-hemisphere and only about 7,000,000 in the other. The portions of 

 land in the water-hemisphere are the extremity of South America below 



