This relation' of the masses in various col umn, and-consequently of the densities, fol lows from the requirement of the of the expression adepth of compensation') that, at that depth, each element 'of mass is subject to equal pressures from all directions. In order that this may be true the vertical pressures, due to gravity, on the various units of area at that depth mast be the same , The geodetk evidence now available proves that the condition called isostasy exists in the earth. It is certain that•there is isostatic com pensation continents as a whole, and for their larger features,.and that the compensation is a fair approximation to being complete and perfect. In the same way there is isostatic compensation for the ocean basins.
The evidence indicates that the depths within the earth at which the isostatic compensation is' approximately' complete and at which, there fore, each element of the mass, is subject to equal pressure from all directions, is about 76 miles (122 kilometers). There great in herent difficulties in, determining this depth with accuracy. It is certain, however, that the depth of annpenSation is much less than the radius of the earth. Seventy-six miles is less than ono. fiftieth of the radius. In other words, it is:cer tain that isostatic, compensation may properly be considered: as a surface condition' of the earth, rather than an interior condition.
It has already been indicated in this article how the geodesist and the geologist each reached the idea of isostasy. In the paragraphs de fining the word isostasy it has been indirectly suggested that the physicists might independ ently have. reached the idea if they had con sidered with sufficient care the tendency to re arrangement of the material within an earth composed of heterogeneous material which, is neither perfectly fluid nor absolutely rigid. •
A fourth point of view, stated below, which may be called the engineering point of • view, also leads inevitably to the conclusion that there must be- some isostatie compensation beneath the earth's surface. Consider the continents and great mountain ranges to be loads on the earth's surf ade in the sense that a railroad em bankment across a plain is an added load, and consider the great ocean basins to be excava dons, or negative loads comparable to canals or railroad cuts- On this 'basis compute the strength which the material composing the earth must have in order that the irregular surface of the earth may be maintained unchanged. It then becomes evident that' the necessary strength is far in 'excess of that possessed by the materials of which the earth is made. Under the condition stated, that is without isostatic compensation, even if the whole earth were made of the strongest granite known; the -con tinents must necessarily slump down into: the oceanic basins and partially fill them. With approximate, isostatic compensation the condi tion is one of approximate equilibrium, and the maximum stresses within the earth material are demonstrably within maximum limits which are not far from the known limits of strength of the materials accessible at the earths surface. Thus from an engineering point- of view isostasy furnishes an explanation • of the ob served semi-permanence of continents and oceanic basins.