FLY THE SHEETS, To let. To let them go suddenly.
FLY-UP. A sudden deviation upwards from a sheer line; the term is nearly synonymous with flight.—To fly up in the wind, is when a ship's head comes suddenly to windward, by carelessness of the helmsman.
FLY-WHEEL. The regulator of a machine.
FOAM [Anglo-Saxon, feám]. The white froth produced by the collision of the waves, or by the bow of a ship when acted on by the wind; and also by their striking against rocks, vessels, or other bodies.
FOCAL LENGTH. The distance between the object-glass and the eye-piece of a telescope.
FOCUS. A point where converging rays or lines meet.
FOEMAN. An enemy in war; now used only by poets. One of Falstaff's recruits, hight Shadow, presented no mark to the enemy: "The foeman may with as great aim level at the edge of a pen-knife."
FŒNUS NAUTICUM. Nautical usury, bottomry.
FOG. A mist at sea, consisting of the grosser vapours floating in the air near the surface of the sea. The fog of the great bank of Newfoundland is caused by the near proximity of warm and cold waters. The air over the Gulf Stream, being warmer than that over the banks of Newfoundland, is capable of keeping much more moisture in invisible suspension; and when this air comes in contact with that above the cold water, it parts with some of its moisture, or rather holds it in visible suspension. There are also dry fogs, which are dust held in suspension, as the so-called African dust, which often partially obscures the sun, and reddens the sails of ships as they pass through the north-east trades.
FOG-BANK. A dense haze, presenting the appearance of a thick cloud resting upon the horizon; it is known in high latitudes as the precursor of wind from the quarter in which it appears. From its frequent resemblance to land it has obtained the name of Cape Fly-away.