9fi Professor A. E. H. Love [March 6, 



the surface of the globe into two regions of equal area : the conti- 

 nental region and the oceanic region. [A slide was shown of a map 

 of the world, true in area, spread out on a rectangle, with the curve 

 drawn.] The continental region is continuous and contains all the 

 continents. The oceanic region consists of two separate portions : the 

 basin of the Pacific Ocean, and the basin of the Atlantic and Indian 

 Oceans. This curve may be called the lioundary of tlie continental 

 region. It is a cardinal feature in any geometrical description of the 

 earth's surface. 



The gravitational theory of the figure of the earth asserts that 

 the cause of the inequalities expressed by continental elevation and 

 oceanic depression is deep-seated. It is sometimes abbreviated into 

 the formula " heavier matter under the oceans." The average density 

 of surface rocks is 2*8 times the density of water. The average 

 density of the earth as a whole is 5 • 5 times the density of water. 

 [The numbers were thrown on the screen.] The formula does not 

 mean that the surface rock of density 2 • 8 has been stripped off the 

 parts where the oceans are, but it means that the denser matter 

 lies rather nearer the surface under the oceans, rather deeper down 

 under the continents. The fact that the figure of the earth is a very 

 good sphere shows that the inequalities in the arrangement of the 

 denser and rarer matter are small, or that the distribution of mass is 

 very nearly symmetrical about a centre. If it were exactly sym- 

 metrical, specimens of the material brought from different parts of 

 any spherical surface described around the centre would all have the 

 same density. The spherical surface would be described as a surface 

 of equal density. The notion of surfaces of equal density is important 

 in the description of unsymmetrical arrangements as well as sym- 

 metrical ones. If on any section of the earth, cut right through the 

 middle, tlie points at which a particular density exists could be joined 

 up, a curve would be formed. The different curves answering to 

 different densities would lie one inside another, like isobars on a 

 weather chart. The density at any point inside a particular curve 

 would be greater than the density at any point on the curve, the 

 density at any point outside it would be less. To join up all the 

 points where the same density is found in the cubic space within 

 the earth would require a surface, just as to join up points in a plane 

 section requires a curve. The surface is a surface of equal density. 

 The series of surfaces of equal density within the earth resemble the 

 coats of an onion, but with the differences that arise from the distinc- 

 tion between a geometrical surface and a thin sheet of matter. The 

 thing we know about these surfaces is that they are nearly spherical 

 and nearly concentric. The surface of water resting on the earth 

 must be everywhere at right angles to the direction of gravity. If 

 the surfaces of equal density were concentric spheres, the surface of 

 the ocean would be a sphere concentric with them, and the whole 

 earth would be covered by water. If the surfaces of equal density 



