CURRENT

, a stream or flux of water in any direction. The setting of the current, is that point of the compafs towards which the waters run; and the drift of a current is the rate it runs an hour.

Currents in the sea, are either natural and general, as arising from the diurnal rotation of the earth on its axis, or the tides, &c; or accidental and particular, caused by the waters being driven against promontories, or into gulphs and streights; from whence they are forced back, and thus disturb the natural flux of the sea.

The currents are so violent under the equator, where the motion of the earth is greatest, that they hurry vessels very speedily from Africa to America; but absolutely prevent their return the same way: so that ships are forced to run as far as the 40th or 45th degree of north latitude, to fall into the return of the current again, to bring them home to Europe. It is shewn by Governor Pownal, that this current performs a continual circulation, setting out from the Guinea coast in Africa, for example, from thence crossing straight over the Atlantic ocean, and so setting into the Gulph of Mexico by the south side of it; then sweeping round by the bottom of the Gulph, it issues out by the north side of it, and thence takes a direction north-easterly along the coast of North America, till it arrives near Newfoundland, where it is turned by a rounding motion backward across the Atlantic again, upon the coasts of Europe, and from thence southward again to the coast of Africa, from whence it set out.

In the streights of Gibraltar, the currents set in by the south side, sweep along the coast of Africa to Egypt, by Palestine, and so return by the northern side, or European coasts, and issue out again by the northern side of the streights. In St. George's Channel too they usually set eastward. The great violence and danger of the sea in the Streights of Magellan, is attributed to two contrary currents setting in, one from the south, and the other from the north sea.

Currents are of some consideration in the art of navigation, as a ship is by them either accelerated or retarded in her course, according as the set is with or against the ship's motion. If a ship sail along the direction of a current, it is evident that the velocity of the current must be added to that of the vessel: but if her course be directly against the current, it must be subtracted: and if she sail athwart it, her motion will be compounded with that of the current; and her velocity augmented or retarded, according to the angle of her direction with that of the current; that is, she will proceed in the diagonal of the parallelogram formed according to the two lines of direction, and will describe or pass over that diagonal in the same time in which she would have described either of the sides, by the separate velocities.

For suppose ABDC be a parallelogram, the diagonal of which is AD. Now if the wind alone would drive the ship from A to B, in the same time as the current alone would drive it from A to C: Then, as the wind neither helps nor hinders the ship from coming towards the line CD, the current will bring it there in the same time as if the wind did not act: And as the current neither helps nor hinders the ship from | coming towards the line BD, the wind will bring it there in the same time as if the current did not act: Therefore the ship must, at the end of that time, be found in both those lines, that is, in their meeting D: Consequently the ship must have passed from A to D in the diagonal AD. See Composition of Forces.

See the Sailing in Currents largely exemplified in Robertson's Navigation, vol. 2, book 7, sect. 8.