94 Professor A. E. H. Love [March 6, 



seismology. The very great rigidity of the matter within the earth is 

 doubtless due to the very great pressure to which this matter is 

 subjected. 



The detection of the ellipticity of the meridians requires rather 

 refined means of observation, because it is very nearly the same for 

 the general mass of the earth as it is for the waters of the ocean; but 

 there are other inequalities of the surface, manifested by the elevation 

 of the continents and the depression of the ocean basins, which are 

 much more obvious. For information in regard to them we have re- 

 course to maps, to observations of the heights of places above sea 

 level, and to soundings. [A map of the world on Mercator's projec- 

 tion was shown and reasons for using other projections indicated.] 

 In order to reduce to a mathematical theory the inequalities in ques- 

 tion, we must begin by getting an arithmetical acquaintance with the 

 facts. Our first question must be as to the sizes of the inequalities. 

 The height of the highest mountain is between five and six miles, the 

 greatest depth yet sounded anywhere in the ocean is less than six 

 miles. Thus the amounts of these inequalities are less than those 

 answering to ellipticity of the meridians. But the corresponding 

 gradients are steeper. On very many coasts the gradient from the 

 shore to very deep water (2000 fathoms) is about 1 in 150. The 

 ellipticity of the meridians gives a maximum gradient, estimated by 

 rate of descent towards the centre, of 1 in 800. Our next question 

 must be as to the sizes of the depressed and elevated portions of the 

 surface. A map on an equal area projection shows tliat much the 

 greater portion of the surface is covered by water. [Maps of two 

 separate hemispheres, each true in area, were shown by slides.] The 

 great expanse of the ocean hides from us many of those features of 

 the somewhat irregular surface of the earth which are partially mani- 

 fested in the continental elevations and oceanic depressions. This 

 surface projects beyond the spheroid appropriate to the rotation in 

 some places, in others it runs inside it. AVhere it projects we say 

 there is elevation ; where it runs inside we say there is depression. 

 Depression does not imply concavity. The surface is almost every- 

 where convex, thougli it is flattpr in some parts than in others. The 

 oceans rest on the depressions and extend upwards over parts of the 

 elevations, and it is the shape of the parts that are covered by water, 

 much more than the shape of the mountainous continental surface, 

 that really determines the shape of the earth. 



Our next question must be as to the amounts of the area of the 

 surface of the earth which are at various heights above or depths 

 below the level of the sea. The most important information is 

 summarised in this table (p. 95). [Thrown on screen.] 



Since only about 2 per cent, of the total surface is more than 

 6000 feet above the level of the sea we need not pay much attention 

 to mountains, but we must pay great attention to the depth of the sea 

 in different parts. It is remarkable that although the depth exceeds 



