24 THE OCEAN 



and condition of the materials of which it is 

 composed. On the other hand, it is known 

 that the rocky crust is heterogeneous in 

 composition to the depth of several miles. 



The gravitational attraction of emerged 

 land causes a heaping-up of ocean waters 

 along continental shores. In consequence of 

 this it has been stated that the waters of the 

 Mid-Indian Ocean are lowered fully 1500 

 feet by the attraction of the elevated regions 

 of the Himalayas. More recent calculations, 

 however, show that the total deformation of 

 the ocean's surface from that which would be 

 assumed by a true spheroid of revolution is 

 not likely to be more than 300 or 400 feet. 

 Other causes, such as difference of barometric 

 pressure, the action of winds, temperature, 

 evaporation, precipitation of rain, and the 

 inflow of rivers, all tend to alter the level of 

 the ocean, the surface of which is really a 

 very complicated one, and must at some points 

 be further removed from the geometrical centre 

 of the earth than at others. It is to this 

 complicated surface of the ocean (or of the 

 geoid) that all measurements of height and 

 depth are referred. 



Areas at different Depths. — The land-surface 

 of the globe covers about 57 millions and 

 the water-surface about 140 millions of 

 English square miles : in other words 29 



