10 



PHYSIOGKAPHIC GEOLOGY. 



1 -300th (accurately, s^.^) shorter than from the centre to the equator. 

 The earth's equatorial radius being 3,963 miles, the polar is about 13£ 

 miles less (exactly 13.2465 miles). 



This is a fact of prime importance in geology, and an appropriate introduction to the 

 science, inasmuch as it is the most obvious proof that the earth has a history, or has 

 been in course of progress under secondary causes; for this flattening is in amount just 

 that which the revolution at its actual rate would produce in a liquid globe having the 

 size and density of the earth. 



(2.) General subdivisions of the surface. — Proportion of Land 

 and Water. — In the surface of the sphere there are about 8 parts of 

 water to 3 of dry land, or, more exactly, 275 to 100 = 5 2 :3 2 . 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 equalling that of the water ; while it is 

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

 tenth (2-21ths) in the south-temperate. 



Out of the 197,000,000 of square miles which make up the entire surface of the globe, 

 144,500,000 are water, and 52,500,000 land. In the northern hemisphere the land covers 

 38,900,000 square miles; in the southern, 13,600,000 square miles. 



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



Fig. 1. 



15 o 15 15 ~0 



all the land of the globe, and the other be almost wholly water. The 

 annexed map represents the two hemispheres. 



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. The 

 portions of land in the water-hemisphere are the extremity of South 

 America below 25° S., and Australia, together with the islands of the 



