A strong south-west or north-west wind invariably raises the tides to an unusual height along the east coast of England and the Channel. Smeaton ascertained by experiment that in a canal four miles in length, the water was kept up four inches higher at one end than at the other, merely by the action of wind along the canal; and Rennell informs us that a large piece of water, ten miles broad, and generally only three feet deep, has by a strong wind had its waters driven to one side, and sustained so as to become six feet deep, while the windward side was laid dry. He also observes, “As water, when pent up so that it cannot escape, acquires a higher level, so, in a place where it can escape, the same operation produces a current, and this current will extend to a greater or less distance according to the force by which it is produced.” Currents flowing alternately in opposite directions are also occasioned by the rise and fall of the tides. The effect of this cause is, as before observed, in estuaries and channels between islands. Evaporation by solar heat is another cause of oceanic currents, of which the great current setting through the Straits of Gibraltar into the Mediterranean, is a remarkable example. A stream of colder water always flows from the Black Sea into the Mediterranean. It must happen in many other parts of the world that large quantities of water, raised from one tract of the ocean by solar heat, are carried to some other, where the vapour is condensed, and falls in the shape of rain, and this, in flowing back again to restore equilibrium, will cause sensible currents. There is still another way in which heat and cold must occasion great movements in the ocean; a cause to which, perhaps, currents are principally due. It is now ascertained that there is in sea water no point, as in fresh water, at which an increase of cold causes the fluid to begin again to expand. In the ocean, therefore, whenever the temperature of the surface is lowered, condensation takes place, and the superficial water having its specific gravity increased, falls to the bottom, upon which lighter water rises immediately, and occupies its place. When this circulation of ascending and descending currents has gone on for a certain time in high latitudes; the inferior parts of the sea are made to consist of colder or heavier fluid than the corresponding depths of the ocean between the tropics. If there be a free communication, if no chain of submarine mountains divide the polar from the equatorial basins, a horizontal movement will arise by the flowing of colder water from the poles to the equator, and there will then be a reflux of warmer superficial water from the equator to the poles. A well-known experiment has been adduced to elucidate this mode of action in explanation of the “trade winds.” If a long trough, divided in the middle by a sluice or partition, have one end filled with water, and the other with quick