Water moves by its weight. Unless there is difference of level, there is no motion. The resistance is from the bottom according to its roughness, and the _vis inertiæ_ is felt at the top--thus the greatest speed is at about two-thirds of the depth; here there is no difference of level, nor is the water acted on superficially by any propelling power. There is no prevalence of winds to account for a current at the surface. So great is the momentum of the stream, that, unlike the currents of the Dardanelles, it is neither accelerated by favourable winds, nor even retarded by adverse storms. The idea of an over-current running against an under-current is so opposed to all experience, that to be admissible, proofs would be required, and it could never be received as an hypothesis to account for an unexplained phenomenon. Thus, the theoretical explanations utterly fail; yet there is action without agent, momentum without motor, currents without winds or declivity, and a vessel constantly filling without escape or overflow. A mighty river rushes over its bed; but this river is not moved by its weight; it runs on a dead level[26] to the sea it reaches from the fountain whence it springs.--That fountain is the ocean itself! No wonder that this should be the first of ancient mysteries, and the last to be explained. Before I had discarded the idea of an under-current, or had discovered the insufficiency of the evaporation to account for the indraught, I was sitting on Partridge Island, (a small rock within the Straits,) and gazing with astonishment at the enormous mass of water running by me, when the question occurred to me, _what becomes of the salt_? If the water evaporate, the salt remains; here then is the sluice of a mighty salt-pan--where is the produce? This has been going on for thousands of years; is there a deposit of salt at the bottom? If so, why have the abysses of the Mediterranean not been filled up? But salt is not deposited; how then is the Mediterranean not become brine? Then I saw that the evaporation would not account for the indraught, and before I descended from that rock, I had solved the problem. That solution is--an under-current produced by a difference of specific gravity between the water of the Mediterranean and the ocean. If you take two vessels, and fill one with fresh water, and the other with salt, or the one with sea-water at its ordinary charge of 1030, and the other with sea-water of higher specific gravity, such as would result from evaporating a portion of it, say 1100, and colour differently the water in the two vessels, and then raise a sluice between them, you will instantly have two currents established in _opposite directions_. In fact, you produce currents of water, like currents of wind, by the converse of rarefaction.