LOG 



LOG 



leAions: and henceforward it became famous, beiiij^ taken 

 notice of both by our own authors, and by foreigners; as 

 by Guntcr in 1623 ; SncUius, in 1624 ; Metius, in i')3i ; 

 Oughtrcd, in 16,^3; llerigonc, in 1634; SaltonftaU, in 

 1636; Norwood, in 1637; rournier, in 1643; and a!- 

 moll by all the fuccccding writers on i.avigation of every 

 conntr)'. See Marlm Survevou. 



Log, to heave the, as they call it, they throw it into the 

 water, on the Ice-fule, letting it run, till it comes without 

 the eddy of tlic fhip's wako ; then one, holding a half- 

 minute glafs, turns it up jull as thi; firll knot, or the mark 

 from which the knots begin to be reckoned, turns off the 

 reel, or palFes over the ftern. As foon as the glafs is out, 

 the reel is Hopped, and the knots run off are told, and their 

 parts ellimated. 



It is ufual to heave the log once every hour in (liips of 

 war and Eaft Indiaraen ; and in all other veffels, once in two 

 hours ; and if at any time of the watch the wind has in- 

 creafed or abated at the intervals, fo as to afcft the ihip's 

 velocity, the ofiicer generally makes a fuitable allowance for 

 it at the clofe of the watch. 



The log is a very precarious way of computing, and mud 

 always be correfted by experience and good fenfe ; there 

 being a great deal of uncertainty in the yawing of the 

 (hip going with the wind aft, or upon the quarter, in the 

 heaving of it, by its coming home, or being drawn after the 

 (hip, on account of the friction of the reel, and lightnefs 

 of the log, in the courfe of the current, and in the ftrength 

 of the wind, which feldom keeps the fame tenor for two 

 hours together ; which is the interval between the times of 

 ufmg the log, in (hort voyages, though in longer ones they 

 heave it every hour. Yet is this a much more exaiit way of 

 computing than any other in ufe ; much preferable cer- 

 tainly to that of the Spaniards and Portuguefe, whogueffed 

 at the fliip's way, by the running of the froth or water by the 

 fhip's fide ; or to that of the Dutch, who ufed to heave a chip 

 overboard, and to number the paces they walk on the deck, 

 while the chip fwims between any two marks, or bulk-heads 

 on the fide. 



The above-mentioned errors, and particularly the log's 

 being fubjett to drive with the motion which the water may 

 have at its furface, 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 confidered by writers, 

 and many methods have been propofed to remove, or at 

 lead to leff n them. The late M. Bouguer propofed a 

 method, which has been thought deferving of particular at- 

 tention, in the Mem. Acad. Sc. 1747; afterwards in his 

 Treatife on Navigation, publifhed at Paris in 1753, and fince 

 reprinted in 1760, by the Abbe de la CaiUe. For this pnr- 

 pofe, take for the lo^ a conical piece of wood, which fix to 

 the log4ine paffed through or along its axis, at about forty, 

 fifty, or fixty, or more feet, from one end ; and to this end 

 fix the diver, which is a body formed of two equal fquare 

 pieces of tin, or of thin iron plate, fixed at right angles to 

 one another along their diagonals ; and its fize fo fitted to 

 that of the cone, that the whole may float. A cone of three 

 inches diameter in the bafe, and of fix inches in the flant 

 height, is propofed by M. Bouguer to fuit a diver made 

 of plates about 9 J inches fquare; the interfeftion 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 ufed in Britifh dock-yards, when 

 formed into a cone of the above dimenfions, will float a 

 diver made of ftout tin plates, one fide of the fquare being 

 9J inches. Such a diver weighing 1 3 lb. av<«)irdupoife, re- 

 quired to float it, a cone of five inches diameter, and twelve 



inches on the flant fide, fo as the point of the cone, whish 

 was made of hght lir, fhonld juft appear abave the water. 

 Now luppofing one fide of Inch a fquare tin diver to be 

 about ten inches, and made of plates only two-tiiirds of the 

 tiiicknefs of the former, fuch a dii»er would weigh, with 

 its folder, about twenty ounces, and can be floated by a 

 light fir cone of four inches diameter in the bafe, and ten 

 inches in the flant height or length ; and fnch a compound - 

 log might, perhaps, be found on trial to be- affeiled by about 

 as much again a"; that propofed by M. Bouguer, and con- 

 fequently the difference between the numbers given by the 

 common log and compound log, mutt be augmented by two- 

 thirds of itii If, for the ncceffary corrcAion, as below. When 

 the compound log of Bouguer, above delcribtd, is hove 

 overboard, the diver will fink too deep to be much affeCted 

 by the current or tnotion of the water at the furface ; and 

 the log will thereby 'teep more ileadily in the place whefe 

 it firii fell ; and confequently, the knots run off the rtel 

 will fhew more accurately the (hip's rate of failing. A» 

 the common log is affefted by the whole motion of the cur- 

 rent, fo this compound log will feel only a part thereof, 

 viz. fuch a part nearly as the refiftancc of the cone is of 

 the refiftance of the diver : then the refiftances of the above 

 cone and diver are abaut as 1 to 5 ; and confequently this 

 log will drive but one-tifth part of what the common log 

 would do ; and fo the fliip's true run will be affeftcd by 

 one-fifth part only of the motion cf the waters. To ob- 

 tain the true rate of failing, it will be proper to heave al- 

 ternately hour and hour, tlie common log, and this com- 

 pound 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 fhip's 

 true rate of failing, at the middle time between the, hours 

 when thefe logs were hove. The corrcftton is additive, 

 when the compound log's xun is the greateft, otherwifc 

 it is fubtradtive. To find the courle made good : increafe 

 the obferved angle between the log-lines by one-fourth 

 part ; and this gives the correction to be applied to the ap- 

 parent courfe, or the oppofite of that fliewn by the common 



log ; the correiEtion is to be applied to the < j^r, {■ of the 



apparent courfe, when the bearing of the common log is 



to the < • 1 , f °f ^^^ compound log. Or thus ; the 



lengths run oft both logs, together with their hearings, 

 being known ; in a card or compafs apply the knots run off, 

 taken from a fcale of equal parts along their refpedive 

 bearings, from the centre ; join the ends, and in this line 

 produced, on the fide 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, wifl (hew the true 

 courfe and diltance made good. When a current, fuch as 

 a tide, runs to any depth, the velocity of that current may 

 be much better afcertained by the compound log than by the 

 common one, provided the diver does not defeend lower 

 than the run of the current ; for as thofe fliips which are 

 deepell immcrged, drive fafteft with the tide ; fo the diver, 

 by being aited on below, as well as the log on the furface, 

 their joint motion will give the total effect to the current's 

 motion better than what could be derived from the motiou 

 at the furface only. Alfo by fuch a compound log, the 

 depth to which any current runs, may be eafily tried. Ro- 

 bertfon's Nav. book ix. (J I. 



We have an account in the Voyage to the North Pole, 

 p 97, of two other logs, which were tried by captain 

 Phipps : one hivented by Mr. Ruffel, the otiier by Foxon ; 

 both conllruded upon this principle, that a fpiral, in pro- 

 ceeding 



