393. Supposing these changes of level between the sea and land to be sufficiently ascertained, the supposition which at first occurs is, that the motion has been in the sea rather than in the land, and that the former has actually descended to a lower level. The imagination naturally feels less difficulty in conceiving, that an unstable fluid like the sea, which changes its level twice every day, has undergone a permanent depression in its surface, than that the land, the _terra firma_ itself, has admitted of an equal elevation. In all this, however, we are guided much more by fancy than reason; for, in order to depress or elevate the absolute level of the sea, by a given quantity, in any one place, we must depress or elevate it by the same quantity over the whole surface of the earth; whereas no such necessity exists with respect to the elevation or depression of the land. To make the sea subside 30 feet all round the coast of Great Britain, it is necessary to displace a body of water 30 feet deep over the whole surface of the ocean. The quantity of matter to be moved in that way is incomparably greater than if the land itself were to be elevated; for though it is nearly three times less in specific gravity, it is as much greater in bulk, as the surface of the ocean is greater than that of this island. 394. Besides, the sea cannot change its level, without a proportional change in the solid bottom on which it rests. Though there be reason to suppose that such changes in the bottom do actually take place, yet they are probably much slower and more imperceptible than those which we are here considering. It is evident, therefore, that the simplest hypothesis for explaining those changes of level, is, that they proceed from the motion, upwards or downwards, of the land itself, and not from that of the sea. As no elevation or depression of the sea can take place, but over the whole, its level cannot be affected by local causes, and is probably as little subject to variation as any thing to be met with on the surface of the globe. 395. Other observations, however, made on different shores from the preceding, give greater certainty to this conclusion, and make it clear, that the motion or change which we are now treating of is not to be ascribed to the sea itself. The observations just mentioned prove, that the level of the North Sea is lower now than it was heretofore; but it appears, that in the Mediterranean, the opposite takes place. Very accurate observations made by MANFREDI, render it certain, that the superficies of the Hadriatic was higher about the middle of the last century, than toward the beginning of the Christian era.