190 The Realm of Nature CHAP. 



above the general level of the Earth, and composed of sub- 

 stances of different density, attract the water by gravity 

 toward themselves, and thus prevent the uniform action 

 of the central force, much as the sides of a tumbler attract 

 the contained water by cohesion and heap it up slightly at 

 the edges. The amount of distortion in the hydrosphere is 

 as difficult to determine as the form of the Earth itself 

 ( 83), and must be found in the same way. It was shown 

 by the survey of India that the sea-surface is 300 feet nearer 

 the centre of the Earth at Ceylon than it is at the Indus 

 delta, where the attraction of the Himalayas comes into 

 play. According to Professor Hull's estimate, the attraction 

 of the Andes is sufficient to raise the level of the sea more 

 than 2000 feet higher on the west coast of South America 

 than at the Sandwich Islands. The rocks beneath the bed 

 of the ocean are, however, believed to be of greater density 

 than those composing continents, and therefore their attrac- 

 tion on the sea should to a large extent counter-balance 

 that of the land. In any case the sea -surface is un- 

 doubtedly not level in any strict sense, and all comparisons 

 of height and depth of distant places are shadowed by 

 uncertainty. 



253. Volume of Oceans and Continents. The most 

 logical datum-level is the mean surface of the lithosphere, 

 the surface which would be produced if the heights were all 

 smoothed down and the hollows rilled up uniformly to pro- 

 duce the geoid. The amount of distortion of the sea- 

 surface must be ascertained, more soundings must be made 

 in many parts of the ocean, and the yet unknown regions 

 surrounding the north and south poles must be explored 

 and surveyed before the position of this ideal surface can 

 be found with certainty. A fair approximation to it has, 

 however, been made in an exhaustive estimate by Dr. 

 John Murray of the area of all the land and of all the oceans 

 lying between certain limits of height and depth. 1 From 

 these areas he calculated the total volume of the land which 

 projects above, and of the oceanic hollows which extend 

 beneath sea -level. The land is estimated to occupy 

 55,000,000 square miles, and its average height is 



