LOG-LINE, a little cord or line, fastened to the log by means of two legs, one of which passes through a hole at the corner, and is knotted on the opposite side, whilst the other leg is attached to the arch by a pin fixed into another hole, so as to draw out occasionally. By these legs the log is suspended in equilibrio; and the line thus annexed to it is wound round a reel, fixed for that purpose in the gallery of the ship.
The use of the log and log-line is to keep account and make an estimate of the ship's way or distance run, which is done by observing the length of fine unwound in half a minute's time, told by a half-minute glass; for so many knots as run out in that time, so many miles the ship sails in an hour. Thus, if there be four knots veered out in half a minute, the ship is computed to run four miles an hour. The author of this device for measuring the ship's way is not known; and no mention of it occurs till the year 1607, in an East India voyage published by Purchas; but from that time its name occurs in other voyages amongst his collections, and henceforward it was noticed both by our own authors and by foreigners; as by Gunter in 1623, Snellius in 1624, and by almost all the succeeding writers on navigation.
The log is a very precarious mode of computation, and must always be corrected by experience, much uncertainty arising from the motions of the ship, the winds of variable force, the friction of the reel, and the lightness of the log in the course of the current. See NAVIGATION.