TACK about, in -the sea-language, is to turn the ship about, or bring her head about, so as to lie the contrary way. In order to explain the theory of tacking a ship, it may be necessary to premise a known axiom in natural philosophy, "that every body will persevere in a state of rest, or of moving uniformly in a right line, unless it be compelled to change its state by forces impressed ; and that the change of motion is proportional to the moving force impressed, and is made ac cording to the right line in which that force is exerted.' By this principle it is easy to conceive how a ship is compell ed to turn in any direction, by the force ofthe wind acting upon her sail in hori zontal lines. For the sails may be so ar ranged as to receive the current of air ei ther directly, or more or less obliquely ; hence the motion communicated to the sails must of necessity conspire with that of the wind upon their surfaces. To make the ship tack, or turn round with her head to the it is therefore ne cessary, after she has received the first impression from the helm, that the head sails should be so disposed as to diminish; the effort of the. wind, in the first instant
oilier motion, and that the whole force of the wind should be exerted on the after sails, which, operating on the ship's stem, carries it round like a 'weathercock. But since the action of the after sails, to turn the ship, will unavoidably cease when her head points to the wind-ward, it then becomes necessary to use the head sails, to prevent her from falling off; and re, turning to her former situation. These are accordingly laid aback on the lee-side, to push the vessel's fore part towards the appointed side, till she has fallen into the line of her course thereon, and fixed her sails to contbrm with that situation.