THE PLAN OF THE EARTH AND ITS CAUSES. 



377 



THE CAUSE OF THE TETRAHEDRAL PLAN. 



The statement that the elevations of the lithosphere are tetrahedral 

 in arrangement is not a hypothesis, but a simple statement of geo- 

 graphical fact. Is the fact a mere coincidence? On the contrary, there 

 are good reasons why the earth should acquire such a tetrahedroid 

 symmetry. When the earth solidified it would (neglecting the influ- 

 ence of rotation) have contracted into a spherical shape. It would have 

 tended to acquire this form because the sphere is the body which 

 incloses the greatest volume for a given surface. But as the earth 

 contracts it tends to acquire a shape in which there is a greater surface 

 in proportion to its bulk. Now, among the regular geometrical figures 



Fig. 6. 



STEINHAUSER'S MAP, SHOWING VARIATION IN ATTRACTION OP GRAVITY, AS INDICATED BY LENGTH OP 

 THIS SECOND-BEATING PENDULUI.l. O = LENGTH IN EQUATORIAL BELT; l-5 = NOS. OP MILLIMETRES BY 

 WHICH PENDULUM HAS TO BE LENGTHENED IN ORDER TO BEAT SECONDS AT DIFFERENT LATITUDES. 



with approximately equal axes the tetrahedron is that which contains 

 the smallest volume for a given surface. Hence every hard-shelled 

 body which is diminishing in size, owing to internal contraction, is con- 

 stat tly tending to become tetrahedral in form. Fairburn's experiments 

 (quoted by Green) illustrate this tetrahedral collapse for short tubes; 

 and that it is considered probable by some geodists is shown by the 

 following quotation from E. D. Preston : 



"Nothing is more in accordance with the action of physical laws than 

 that the earth is contracting in approximately a tetrahedral form. 

 Given a collapsing homogeneous spherical envelope, it will assume that 

 regular shape which most readily disposes of the excess of its surface 

 dimensions, or, in other words, the shape that most easily relieves the 



