PRESIDENTIAL ADDRESS. 4.29 
enclosed patches of deep water. Two of these arein the Mediterranean, one in the 
Arctic Ocean, and others are in the Gulf of Mexico and the Caribbean Sea. The Red 
Sea, the Mediterranean, and the Arctic Ocean belong to the continental region, and 
so do the Gulf of Mexico and the Caribbean Sea. At this depth Asia and North 
America are joined across Behring’s Strait, and Europe is joined to North America 
across the British Isles, Iceland, and Greenland; Australia is joined to Asia 
through Borneo and New Guinea, and the Australasian continental region nearly 
reaches the Antarctic region by way of New Zealand. At this depth also South 
America does not taper to the south, but spreads out, and is separated from the 
Antarctic region by a very narrow channel. By going down to great depths our 
problem is very much simplified. We find that the surface of the earth can be 
divided into continental and oceanic regions of approximately equal area by a 
curve which approaches a regular geometrical shape. By smoothing away the 
irregularities we obtain the curve shown in fig. 38, which exhibits the surface as 
divided up into a continuous continental region and two oceanic regions—the 
basin of the Pacific Ocean and the basin of the Atlantic and Indian Oceans. We 
may take our problem to be this: to account on dynamical grounds for the 
Pacific 
Ocean 
Indian 
Fig.3. 
separation of the surface into a continental region and two oceanic regions which 
are approximately of this shape. 
The key of the problem was put into our hands four years ago by Jeans in his 
theory of gravitational instability. If there are any differences of density in 
different parts of a gravitating body, the denser parts attract with a greater force 
than the rarer parts, and thus more and more of the mass tends to be drawn 
towards the parts where the density is in excess, and away from the parts where it 
is in defect. In every gravitating system there is a tendency to instability. Ina 
body of planetary dimensions this tendency, if it were not checked, would result 
in a concentration of the mass either towards the centre or towards some other 
part. But concentration of the mass means compression of the material, and it 
cannot proceed very far without being checked by the resistance which the material 
offers to compression. There ensues a sort of competition between two agencies: 
gravitation, making for instability, and the elastic resistance to compression, 
making for stability. Such competing agencies are familiar in other questions 
concerning the stability of deformable bodies. A long thin bar set up on end 
tends to bend under its own weight. A steel knitting-needle a foot long can 
stand up; a piece of thin paper of the same length would bend over. In order 
that a body may be stable in an assigned configuration there must be some 
relation between the forces which make for instability, the size of the body, 
and the resistance which it offers to changes of size and shape. In the case 
of a gravitating planet we may inquire how small its resistance to compres- 
sion must be in order that it may be unstable, and, further, in respect of what 
types of displacement the instability would manifest itself. If we assign the 
constitution of the planet, the inquiry becomes a definite mathematical problem. 
The greatest difficulty in the pioblem arisesfrom the enormous stresses which 
are developed within such a body as the earth by the mutual gravitation of 
