A very interesting point may now be referred to, because it seems confirmatory of this origin of the great ocean basins. In Mr. Osmond Fisher's work it is explained how the variations in the force of gravity, at numerous points all over the world, have been determined by observations with the pendulum, and also how these variations afford a measure of the thickness of the solid crust, which is of less specific gravity than the molten interior on which it rests. By this means a very interesting result was obtained. The observations on numerous oceanic islands proved that the sub-oceanic crust was considerably more dense than the crust under the continents, but also thinner, the result being to bring the average mass of the sub-oceanic crust and oceans to an equality with that of the continental crust, and this causes the whirling earth to be in a state of balance, or equilibrium. Now, both the thinness and the increased density of the crust seem to be well explained by this theory of the origin of the oceanic basins. The new crust would necessarily for a long time be thinner than the older portion, because formed so much later, but it would very soon become cool enough to allow the aqueous vapour of the atmosphere and that given off through fissures from the molten interior to collect in the ocean basins, which would thenceforth be cooled more rapidly and kept at a uniform temperature and also under a uniform pressure, and these conditions would lead to the steady and continuous increase of thickness, with a greater compactness of structure than in the continental areas. It is no doubt to this uniformity of conditions, with a lowering of the bottom temperature throughout the greater part of geological time, till it has become only a few degrees above the freezing-point, that we owe the remarkable persistence of the vast and deep ocean basins on which, as we have seen, the continuity of life on the earth has largely depended. There is one other fact which lends some support to this theory of the origin of the ocean basins--their almost complete symmetry with regard to the equator. Both the Atlantic and Pacific basins extend to an equal distance north and south of the equator, an equality which could hardly have been produced by any cause not directly connected with the earth's rotation. The polar seas which are coterminous with the two great oceans are very much shallower, and cannot, therefore, be considered as forming part of the true oceanic basins. WATER AS AN EQUALISER OF TEMPERATURE The importance of water in regulating the temperature of the earth is so great that, even if we had enough water on the land for all the wants of plants and animals, but had no great oceans, it is almost certain that the earth could not have produced and sustained the various forms of life which it now possesses.