OF THE LOG-LINE AND HALF-MINUTE GLASS, AND HOW TO CORRECT THE DISTANCE GIVEN BY THEM. TH HE log is a flat piece of wood like a flounder, or of the figure of a quarter of a circle, having its circular fide loaded with lead fufficient to make it fwim upright in the water .To this log is faftened a long line of about 150 fathoms, called the log-line, which is divided into certain equal spaces, called knots, each of which ought to bear the fame proportion to a nautical mile (60 of which make a degree) that half a minute does to an hour, that being the time allowed for the experiment. They are called knots, becaufe at the end of each of them there is a piece of twine with knots in it, reeved between the ftrands of the line; these pieces of twine fhew how many knots run out in half a minute, and confequently the fhip's rate of failing per hour, Mr. NORWOOD, and feveral other able mathematicians, have found that a degree of a great circle upon the earth contains about 367,200 English feet, therefore a nautical mile being part of 357,200 feet, that is, 6120 feet, and fince half a minute is part of an hour, the length of the knot on the log-line ought to be the To part of 6120 feet, or 51 feet. (In the requifite Tables publithed in 1802, the fea mile is accounted 6078 feet.) But as for the most part, the fhip's way is found, by experience, to be really more than that given by the log, and as it is fafer to have the reckoning before the ship than after it, therefore 50 feet may be taken as the proper length of each knot, and these knots fubdivided into ten fathoms, each of five feet, which is certainly the beft adapted for practice, and will correfpond with all the tables and inftruments used in navigation, as they are decimally divided, and confequently, the ship's run determined with greater cafe and certainty. But fome experienced commanders find, that the allowing 50 feet to a knot generally makes the ship a-head of the reckoning; and to avoid danger moftly divide the log-line into knots of 7 or 7 fathoms of 6 feet each, to correfpond with a glafs that runs 28 feconds. Others again divide the feconds the glass runs by 4, and take the quotient for the diftance in fathoms between the knots; which laft method I have used for 40 years, and always found it anfwered; but certain it is, that whatever length the knots are, the most convenient way is to divide them into tenths. In hot or dry weather, the glass runs out fafter than in mcift or rainy rainy weather; therefore care fhould be taken to try what number of feconds the glafs runs. The knots commonly begin to be counted at the diftance of 10, 12, or 15 fathoms from the log, according to the largenefs of the fhip, that fo the log may be out of the ship's wake when it is thrown overboard before they begin to count, let the eddies fhould fuck the log after the ship; and for the most ready difcovery of this point of commencement, there is commonly faftened at it a piece of red rag; that part of the line between the red rag and the log is called the ftray-line. The log and log-line being duly prepared and hove overboard from the lee quarter, and the line veered out (by the help of a reel, which turns easy, and about which it is wound) as faft as the log will carry it away, or rather as faft as the fhip fails from it, will fhow how fast the hip has failed in the given time, or rate of failing per hour. The experiment for finding the velocity of the fhip is called heaving the log. Care fhould be taken to veer out the line as fast as the log takes it, for if the log is left to turn the reel of itfelf, the log will come home and deceive you in the reckoning. In King's fhips, India thips, and fome others, the log is hove every hour, but in coafters, and thofe ufing fhort voyages, every two hours. Here the fhip is fuppofed to move with equal velocity between the times of trying the experiment. But if the gale has not been the fame during the whole hour, or time between heaving the log, or if there have been more fail fet, or any handed, that fo the fhip has run more or lefs in any part of the hour than fhe did at the time of the experiment; or if it should fall little or more wind at that time, there must be allowance made for it according to the difcretion of the artift: Sometimes, too, when the fhip is before the wind, and a great fea fetting after her, it will bring home the log; in fuch cafes it is customary to allow one mile in ten, and lefs in proportion, if the fea be not fo great. Care fhould alfo be taken to measure the log-line pretty often, left it stretch, and deceive you in the distance. The like regard must be had, that the half-minute glass be just 30 feconds, otherwife no account of the fhip's way can be kept; to prove which, if there be no top watch at hand, let a plummet, of any form or weight, be faftened to a filk ftring or thread, with at loop to hang on a small pin or nail aftened in any place, fo that the plummet may fwing freely; let it be 391 inches from the end of the loop to the middle of the plummet, and the plummet caufed to fwing; each of thofe fwings will be a true fecond of time, always counting every time it paffes the perpendicular let fall from the pin, and every time it patles from the perpendicular to the utmoft fwing will be half-a-fecond. How How to correct the Distance given by the Log-Line and Half- The distance given by the log may be wrong on three accounts. viz. by an error in the glafs, an error in the log-line, or an error in both; for correcting of which take the following cafes: CASE I. When the log-line is truly divided, and the glafs is faulty. RULE. Say, as the feconds run by the glafs are to 30 feconds, fo is the diftance given by the log to the true diftance. EXAMPLE. I. Suppofe a fhip runs at the rate of 7 knots in the time the glafs runs out, but measuring the glafs I find it runs 34 feconds; what is the true rate of failing? As 34 30 7,5 6,6 miles, the true diftance failed in an hour. EXAMPLE II. Suppofe a fhip runs at the rate of 6 knots, but measuring the glafs I find it runs only 25 feconds; required the true rate of failing? As 25: 30:: 6,5 7,8 miles, the true distance failed in an hour. CASE II. When the glass is true and log line faulty. RULE. Say, as 50 feet is to the distance measured between knot and knot, fo is the distance run by the log to the true distance. EXAMPLE I. Suppofe a fhip runs at the rate of 6 knots in half a minute, but meafuring the space between knot and knot, I find it to be 56 feet; required the true rate of failing? As 50: 56: 6,25: 7 miles, the true diftance failed in an hour. EXAMPLE II. Suppose a ship runs at the rate of 6 knots in half a minute, but measuring the space between knot and knot, I find it to be only 44 feet; required the true rate of fai'ing? As 50: 44: 6,5: 5,72 miles, the true distance failed in an hour. CASE III. When both the log-line and glafs are faulty. RULE. Multiply thrice the meafured length of a knot by the diftance run by the log, the product divided by 5 times the meafured time of the glass will give the true diftance run. EXAMPLE |