in the Jewish antiquities, a measure which held a quarter of a cab, and consequently five-sixths of a pint. There is mention of a log, 2 Kings vi. 25, under the name of a fourth part of a cab. But in Leviticus the word log is often met with, and signifies the measure of oil which lepers were to offer at the temple after they were cured of their disease. Dr Arthurnot says, that the log was a measure of liquids, the seventy-second part of the bath or ephah, and twelfth part of the hin, according to all the accounts of the Jewish writers.

a sea term, signifying a small piece of timber a, (fig. 3.) of a triangular, sectoral, or quadrantal figure, on board a ship, generally about a quarter of an inch thick, and five or fix inches from the angular point to the circumference. It is balanced by a thin plate of lead, nailed to the arch, or circular side, so as to swim perpendicularly in the water.

LOG-Line, a little cord, or line, about a hundred and fifty fathoms long, fastened to the log by means of two legs a b (fig. 4.), one of which passes through a hole at the corner, and is knotted on the opposite side, while 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 hung in equilibrium; and the line thus annexed to it is wound round a reel (fig. 2.), fixed for Fig. 2. that purpose in the gallery of the ship.

This line, from the distance of about ten, twelve, or fifteen fathoms off the log, has certain knots or divisions, which ought to be at least fifty feet from each other; though it was the common practice at sea not to have them above forty-two feet alunder.

The length of each knot ought to be the same part of a sea mile as half a minute is of an hour; and admitting the measurement of Mr Norwood, who makes a degree on a great circle of the earth to contain 367,200 English feet, or about 69\( \frac{1}{2} \) English statute miles, and therefore \( \frac{1}{60} \)th part of it, or a nautical mile, will be 6120 feet; \( \frac{1}{60} \)th of 6120, or 51 feet, should be the length of each knot. But because it is safer to have the reckoning rather before the ship than after it, therefore fifty feet may be taken as the proper length of each knot. The knots are sometimes made to consist only of forty-two feet each, even in the present practice; and this method of dividing the log-line was founded on the supposition that 60 miles, each of 5000 English feet, made a degree; for \( \frac{1}{60} \) of 5000 is 41\( \frac{2}{3} \), or, in round numbers, 42 feet. Mariners rather than quit the old way, though known to be erroneous, use glasfs for half minute ones, that run but 24 or 25 seconds. They have also used a line of 45 feet to 30 seconds, or a glass of 28 seconds to 42 feet. When this is the case, the distance between the knots should be corrected by the following proportion: as 30 is to 50; so is the number of seconds of the glass to the distance between the knots upon the line. The heat or moisture of the weather has often a considerable effect upon the glass, so as to make it run slower or faster; it should, therefore, be frequently tried by the pendulum in the following manner. On a round nail hang a 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 peg 39\( \frac{1}{2} \) inches, being the length of a second pendulum; then swing it, and count one for every time it passes under the peg, beginning at the second time it passes, and the number of swings made during the time the glass is running out shows the seconds it contains. The line also is liable to relax and shrink, and should therefore be occasionally measured.

The use of the log and 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 line 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 among his collections; and henceforward it became famous, being taken notice of both by our own authors and by foreigners; as by Gunter in 1623; Snel- lius in 1624; Metius in 1631; Oughtred in 1633; Herigone in 1634; Saltonstall in 1636; Norwood in 1637; Pournier in 1643; and almost by all the succeeding writers on navigation of every country.

To Heave the Log, as they call it, they throw it into the water on the lee side, letting it run till it comes without the eddy of the ship's wake; then one holding a half minute glass, turns it up just as the first knot, or the mark from which the knots begin to be reckoned, turns off the reel (fig. 2), or paffes over the stern. As soon as the glass is out, the reel is stopped, and the knots run off are told, and their parts estimated.

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, allowance being made for the wind having increased or abated in the intervals.

The log is a very precarious way of computing, and must always be corrected by experience, there being much uncertainty from the motions of the ship, the winds of variable force, the friction of the reel and lightness of the log in the course of the current. Yet this is a much more exact way of computing than any other in use; much preferable certainly to that of the Spaniards and Portuguese, who guessed at the ship's way by the running of the froth or water by the ship's side; or to that of the Dutch, who used to heave a chip overboard, and to number the paces they walk on the deck while the chip swims between any two marks or bulk heads on the side.

Compound Log. The above-mentioned errors, and particularly the log's being subject to drive with the motion of the water at its surface, whereas the experiment requires it to be fixed in the place where it is when the mark commencing the knots goes off the reel, have been considered, and many methods proposed to remove or to lessen them. M. Bouguer proposed the following method. Take for the log a conical piece of wood, which fix to the log line passed through or along its axis, at about 40, 50, or 60, or more feet, from one end; and to this end fix the diver, which is a body formed of two equal square pieces of tin, or of thin iron plate, fixed at right angles to one another along their diagonals; and its size to fitted to that of the cone, that the whole may float. A cone of three inches diameter in the base, and of six inches in the flant height, is proposed by M. Bouguer to suit a diver made of plates about 9 1/2 inches square; the intersection of the diagonals is joined to the log line, and the loop and peg fixed as in the common log. However, it has been found, that no kind of wood used in British docks yards, when formed into a cone of the above dimensions, will float a diver made of stout tin plates, one side of the square being 9 1/2 inches. Such a diver weighing 1 1/2 lb. avoirdupois, required to float it a cone of five inches diameter and twelve inches on the flant side, so as the point of the cone, which was made of light fir, should just appear above the water. Now, supposing one side of such a square tin diver to be about ten inches, and made of plates only two-thirds of the thickness of the former, such a diver would weigh, with its folder, about 20 ounces, and can be floated by a light fir cone of four inches diameter in the base, and ten inches in the flant height or length; and such a compound log might perhaps be found on trial to be affected by about as much again as that proposed by M. Bouguer; and consequently the difference between the numbers given by the common log and compound log, must be augmented by two-thirds of itself for the necessary correction, as below. When the compound log of Bouguer, above described, is hove overboard, the diver will sink too deep to be much affected by the current or motion of water at the surface, and the log will thereby keep more steadily in the place where it first fell; and consequently the knots run off the reel will show more accurately the ship's rate of sailing. As the common log is affected by the whole motion of the current, so this compound log will feel only a part thereof, viz. such a part nearly as the resistance of the cone is to the resistance of the diver; then the resistances of the above cone and diver are about as 1 to 5; and consequently this log will drive but one-fifth part of what the common log would do; and so the ship's true run will be affected by one-fifth only of the motion of the waters. To obtain the true rate of sailing, it will be proper to heave alternately, hour and hour, the common log, and this compound log; then the difference of their knots run off, augmented by its one-fourth part, is the correction; which applied to the knots of the common log, will give the ship's true rate of sailing at the middle time between the hours when these logs were hove. The correction is additive when the compound log's run is the greatest, otherwise it is subtractive. To find the course made good: increase the observed angle between the log lines by one-fourth part; and this gives the correction to be applied to the apparent course, or the opposite of that shown by the common log; the correction is to be applied to the {right} of the apparent course, when the bearing of the common log is to the {left} of the compound log. Or thus: the lengths run off both logs, together with their bearings, being known; in a card or compass apply the knots run off, taken from a scale of equal parts along their respective bearings from the centre; join the ends; and in this line produced, on the side next the compound log's length, take one-fourth of the interval; then a line drawn from the end, thus produced, to the centre of the card, will show the true course and distance made good. When a current, such as a tide, runs to any depth, the velocity of that current may be much better ascertained by the compound log than by the common one, provided the diver does not descend lower than the run of the current; for as those ships which are deepest immersed, drive fastest with the tide; so the diver, by being acted on below, as well as the log on the surface, their joint motion will give the total effect of the current's motion better than what could be derived from the motion at the surface only. Also, by such a compound log, the depth to which any current runs may be easily tried.

Other LOGS. We have an account in the voyage to the North

PLATE CCXCVII.

Fig. 1. Fig. 2. Fig. 3. Fig. 4.