LOG

, in Navigation, is a piece of thin board, of a sectoral or quadrantal form, loaded in the circular side with lead sufficient to make it swim upright in the water; to which is fastened a line of about 150 fathoms, or 300 yards long, called the Log-line, which is divided into certain spaces, called Knots, and wound on a reel which turns very freely, for the line to wind easily off.

The use of the Log, or Log-line, is to measure the velocity of the ship, or rate at which she runs, which is done from time to time, as the foundation upon which the ship's reckoning, or finding her place, is kept; and the practice is to heave the Log into the sea, with the line tied to it, and observe how much of the line is run off the reel, while the ship sails, during the space of half a minute, which time is measured by a sand-glass made to run that time very exactly. About 10 fathoms of stray or waste line is left next the Log before the knotting or counting commence, that space being usually allowed to carry the Log out of the eddy of the ship's wake.

The using of the Log for finding the velocity of the ship, is called Heaving the Log, and is thus performed: One man holds the reel, and another the halfminute glass; an officer of the watch throws the Log over the ship's stern, on the lee-side, and when he observes the stray line, and the first mark is going off, he cries turn! when the glass-holder instantly turns the glass crying out done! then watching the glass, the moment it is run out he says stop! upon which the reel being quickly stopt, the last mark run off shews the number of knots, and the distance of that mark from the reel is estimated in fathoms: then the knots and fathoms together shew the distance run in half a minute, or the distance per hour nearly, by considering the knots as miles, and the fathoms as decimals of a mile: thus if 7 knots and 4 fathoms be observed, then the ship runs at the rate of 7.4 miles an hour.

It follows, therefore, that the length of each knot, or division of the line, ought to be the same part of a sea mile, as half a minute is of an hour, that is 1/120 th part. Now it is found that a degree of the meridian contains nearly 366,000 feet, therefore 1/<*>0 of this, or a nautical mile, will be 6100 feet; the 1/120 th of which, or 51 feet nearly, should be the length of each knot, or division of the Log-line. But because it is safer to have the reckoning rather before the ship than after it, therefore it is usual now to make each knot cqual to 8 fathoms or 48 feet. But the knots are made sometimes to contain only 42 feet; and this method of dividing the Log-line was founded on the supposition, that 60 miles, of 5000 feet each, made a degree; for 1/120th of 5000 is 41 2/3, or in round numbers 42 feet. And although many mariners find by experience that this length of the knot is too short, yet rather than quit the old way, they use sand-glasses for half-minute ones that run only 24 or 25 seconds. The sand, or halfminute glass, may be tried by a pendulum vibrating seconds, in the following manner: On a round nail or peg, hang a thread or fine string that has a musket ball fixed to one end, carefully measuring between the centre of the ball and the string's loop over the nail 39 1/8 inches, being the length of a second pendulum; then make it swing or vibrate very small arches, and count one for every time it passes under the nail, beginning at the second time it passes; and the number of swings made during the time the glass is running out, shews the seconds in the glass.

It is not known who was the inventor of this method of measuring the ship's way, or her rate of sailing; but no mention of it occurs till the year 1607, in an East-India voyage, published by Purchas; and from that time its name occurs in other voyages in his collections; after which it became famous, being noticed both by our own authors, and by foreigners; as by Gunter in 1623; Snellius, in 1624; Metius, in 1631; Oughtred, in 1633; Herigone, in 1634; Saltonstall, in 1636; Norwood, in 1637; Fournier, in 1643; and almost all the succeeding writers on navigation of every country. Various improvements have lately been made of this instrument by different persons.