LOG

The log, fig. 14, plate V. is generally about a quarter of an inch thick, and five or six inches from the angular point a to the circumference b. It is balanced by a thin plate of lead, nailed upon the arch, so as to swim perpendicularly in the water, with about 2/3 impressed under the surface. The line is fastened to the log by means of two legs a and b, fig. 15, one of which passes thro' a hole a at the corner, and is knotted on the opposite side; whilst the other leg is attached to the arch by a pin b, fixed in another hole, so as to draw out occasionally. By these legs the log is hung in equilibrio, and the line, which is united to it, is divided into certain spaces, which are in proportion to an equal number of geographical miles, as a half minute or quarter minute is to an hour of time.

Plate V

This instrument is employed to measure the ship's course in the following manner: The reel, fig. 16, about which the log-line is wound, being held by one man, and the half-minute glass by another, the mate of the watch at the same time fixes the pin, and throws the log over the stern, which, swimming perpendicularly in the sea, feels an immediate resistance as the ship advances. To prevent the pin from being drawn by the effort of this resistance, the person who heaves the log continually slackens the line over the stern, or quarter, so that it becomes almost streight on the water, and the log continues nearly in the same place where it first alighted, and is considered as fixed therein. The knots are measured from a mark fastened at the distance of 12 or 15 fathoms from the log; the glass is therefore turned at the instant when this mark passes over the stern, and as soon as the glass runs out, the line is accordingly stopped; when the water, acting forcibly on the surface of the log, immediately dislodges the pin, so that the log, no longer resisting the effort of the water, is easily drawn aboard. The degree of the ship's velocity is then readily determined, by examining the number of knots nearest to that part of the line, where it was stopped at the expiration of the glass, as the knots increase in their natural order from the mark above-mentioned. The space comprehended between that mark and the log is used to let the latter be far enough astern, to be out of the eddy of the ship's wake when the glass is turned.

If the glass runs thirty seconds, the distance between the knots should be fifty seet. When it runs more or less, it should therefore be corrected by the following analogy: As 30 is to 50, so is the number of seconds of the glass to the distance between the knots upon the line. As the heat or moisture of the weather has often a considerable effect on the glass, so as to make it run slower or faster, it should be frequently tried by the vibrations of a pendulum. The line, being also liable to relax or shrink from the same cause, ought likewise to be measured, as occasion requires.

It is usual to heave the log once every hour in ships of war and East Indiamen; and in all other vessels, once in two hours; and if at any time of the watch, the wind has increased or abated in the intervals, so as to affect the ship's velocity, the officer generally makes a suitable allowance for it, at the close of the watch.

Reference Works