SAIL. A sail is an expanse of canvas. matting, or other strong material, on which the wind may exert its force and propel the vessel. A sail is extended by means of a mast or yard, or both. It may be of various shapes. and of any size, according to the carrying power of the vessel. A vessel of shallow draught or of beam can bear comparatively little sail; while a vessel of proportionately deep draught and heavily ballasted—as a yacht—or a vessel of great breadth of beam, can carry sail of great area. A sail acts with the greatest power when the wind is directly astern; but it can be applied. though with less strength, when on either beam. The action of the wind nn an oblique sail is a good example of the resolution of forces. See ComwosrrioN AND RESOLUTION OF PoneEs, etc. Let TD, fig. 1, be a ship, PAS its sail, WA the direction of the wind, and let the length of WA represent the pressure of the whd on the sail. WA can be resolved into AB perpendicular to the sail, and BW parallel to it, the latter of which has no effect in pressing on the sail; therefore AB is the effective pressure on the sail. Were the vessel round, it would move hi the direction BA. Let BA be resolved into CA and BC, the former, CA, acting in the direction of the keel or length of the vessel, or in the direction CAD, and the latter per pendicular to it, or in the direction of the breadth. The former pressure, CA, is the only pressure that moves the vessel forward, the other, BC, makes it move sideways. From the form of the vessel, however. this latter force, BC, produces comparatively little lateral motion; any that it does occasion is called leeway. It results, therefore, that with the wind exerting an oblique pressure, the actual progress will be to the power of the wind only as CA to WA. In the east and the Mediterranean, sails arc frequently made of strong matting; but among northern nations, and for ocean navigation, very strong cloth, or canvas, called sail-ctoth, is usually resorted to. It is woven narrow; and the many breadths in the sail are joined by carefully made double seams.
Sails are nearly always either triangulf.r or quadrilateral, but not necessarily equi angular. give greater strength, a strong rope or cord is sewn into the outer edge till round the sail: this rope has eyes in it, to which the various ropes employed in con nection with the sail are fastened. The top of a sail is its head; the bottom, its foot;
and the sides are leeches; the upper corners are termed ear-rings the lower corners of a square sail, and the after lower corner of other sails, clews ; the front lower corner of a fore-and-aft sail is the tack. The ropes from the lower corners, used in tightening the sail against the wind, are the sheets.
The • ail; of a'ship arc either " square" or "fore-and-aft." The square-sails—begin ning from below—are the course, the topsail, the topgallant-sail, the royal, and, though very rarely used, the sky-sc•aper. Each has the name of the mast on which it is set pre fixed, as "fore-topsail," " main-royal," ete. The square-sails are made fast by their beads to yards, the foot being drawn to the extremity of the yard below. Fore-and-aft sails are the spit nke r or driver, extended by the gaff at its bead, boom at its foot, and mast on its fore-leech; the staysails, which are suspended by rings to the stays, and the jibs (q.v.). In it three-masted vessel the Sails of most importance are the main-course, the spanker, the topsails, the fore-staysail, and the jibs, which can usually be all distended to the full with-tot taking wind from each other. In very light winds,when every breadth is of consequence, the area of the sails is increased by setting the stadings•aits, which are oblong sails set on each side of the square-sails, on short booms run out beyond the yards of the latter.
In small craft and boats. the most common sail is a lugsail (see LUGGER), which is a small square-sail, occasionally supplemented by a shoulder-of-mutton (triangu(ar) sail on a shorter mast at the stern. Cutters or sloops carry a large spanker, with a topsail of similar shape, and jilis; some having the power of setting ii large oorse when the wind is astern ; but, it is obvious that the course and spa nker cannot be used together. A schooner uses the same sails as a cutter, except that in one form she carries a square topsail and topgallant-sail on the foremast.
Sails are furnished with rows of short ropes for the purpose of reefing them, when their area is too large for the wind. The effect of a sail is increased by wetting it, as the pores of the canvas close more tightly through the swelling of the hemp.