SAILING, the movement by which a vessel is wafted along the surface of the water, by the action of the wind upon her sails. When a ship changes her state of rest into that of motion, as in advancing out of a harbour, or from her station at anchor, she acquires motion very gradually, as a body which arrives not at a certain velocity till after an infinite repetition of the action of its weight. The first impression of the wind greatly affects the velocity, because the resistance of the water might destroy it; since the velocity being but small at first, the resistance of the water which depends upon it will be very feeble. But as the ship increases her motion, the force of the wind on the sails will be diminished; and, on the contrary, the resistance of the water on the bow will accumulate in proportion to the velocity with which the vessel advances. Thus the repetition of the degrees of force, which the action of the sail adds to the motion of the ship, is perpetually decreasing; while the new degrees added to the effort of resistance on the bow are always augmenting. The velocity is then accelerated in proportion as the quantity added is greater than that which is subtracted; but when the two powers become equal, when the impression of the wind on the sails has lost so much of its force as only to act in proportion to the opposite impulse of resistance on the bow, the ship will then acquire no additional velocity, but continue to sail with a constant uniform motion. The great weight of the ship may indeed prevent her from acquiring the greatest velocity; but when she has attained it, she will advance by her own intrinsic motion, without gaining any new degree of velocity, or lessening what she has acquired. She moves then by her own proper force in vacuo, without being afterwards subject either to the effort of the wind on the sails, or to the resistance of the water on the bow. If at any time the impulse of the water on the bow should destroy any part of the velocity, the effort of the wind on the sails will revive it, so that the motion will continue the same. It must, however, be observed, that this state will only subsist when these two powers act upon each other in direct opposition, otherwise they will mutually destroy one another. The whole theory of working ships depends on this counter action, and the perfect equality which should subsist between the effort of the wind and the impulsion of the water. The effect of sailing is produced by a judicious arrangement of the sails to the direction of the wind. Accordingly the various modes of sailing are derived from the different degrees and situations of the wind with regard to the course of the vessel. See SEAMANSHIP. SAILING also implies a particular mode of navigation, which is formed on the principles and regulated by the laws of trigonometry. Hence we say, Plain Sailing, Mercator's, Middle-latitude, Parallel, and Great-circle Sailing.