LOG. 581 
l»y a pin fixed into another hole, fo as to draw out occa- 
iionally. By thefe legs the log is hung in equilibrio ; and 
the line thus annexed to it is wound round a reel (fig. 4) 
fixed for that purpofe in the gallery of the (hip. The 
French log is a perfect triangle, and is connected to the 
line by more cords, or legs, as at fig. 5. In either cafe, 
the log-line, from the diltance of about ten, twelve, or fif¬ 
teen, fathoms off the log, has certain knots or divifions, 
which ought to be at lea It fifty feet from each other; 
though it was the common practice at fea not to have 
them above forty-two feet afunder. The length of each 
knot ought to be the fame part of a fea-mile as half a 
minute is of an hour; and, admitting the meafurement of 
Mr. Norwood, who makes a degree on a great circle of 
the earth to contain 367,200 Englifh feet, or about 69^ 
Englifh ftatute-miles, the Jyth part of it, or a nautical 
snile, will be 6120 feet; and the °f 6120, or 51 feet, 
Ihould be the length of each knot. But, becaufe it is fafer 
to have the reckoning rather before the (hip than after it, 
therefore fifty feet may be taken as the proper length of 
each knot. The knots are fometimes made to confilt only 
of forty-two feet each, even in the prefent practice ; and 
this method of dividing the log-line was founded on the 
fuppofition that fixty miles, each of 5000 Englifii feet, made 
a degree ; for 7-I-0 of 5000 is 41^, or, in round numbers, 
42 feet. Mariners, rather than quit the old way, though 
known to be erroneous, ufe glaffes for half-minute ones 
that run but 24 or 25 feconds. They have alfo ufed a 
line of 45 feet to 30 feconds, ora glafs of 28 feconds to 42 
feet. When this is the cafe, the didance between the 
knots (liould be corrected by the following proportion : 
As 30 is to 50, fo is the number of feconds of the glafs 
to the diltance between the knots upon the line. The 
heat or moifture of the weather has often a confiderable 
elfedt upon the glafs, fo as to make it run (lower or fader; 
it Ihould, the r efore, be frequently tried by the pendulum 
in the following manner: On a round nail hang a firing 
that has a mufket-ball fixed to one end, carefully mea- 
furing between the centre of the ball and the firing’s loop 
over the peg 397 inches, being the length of a fecond pen¬ 
dulum ; then fwing it, and count one for every time it 
pafles under the peg, beginning at the fecond time it 
pafles: and the number of fwings made during the time 
the glafs is.running out (hows the feconds it contains. 
The line alfo is liable to relax and (brink, and Ihould 
therefore be occafionally meafured. 
The ufe of the log and line is to keep account and make 
an eftimate of the (hip’s way or diltance run ; which is 
dene by obferving the length of line unwound in half a 
minute’s time, told by a half-minute glafs; for fo many 
knots as run out in that time, fo many miles the (hip fails 
in an hour. Thus, if there be four knots veered out in 
half a minute, the (hip is computed to run four miles an 
hour-. 
The author of this device for meafuring the (hip’s way 
is not known ; and no mention of it occurs till the year 
1607, in an Eaft-India voyage publiflied by Purchas; but 
from that time its name occurs in other voyages among 
his Collections ; and thenceforward it became famou$| be¬ 
ing taken notice of both by our own authors and by fo¬ 
reigners ; as by Gunter in 1623; Snellius in 1624; Me- 
tius in 1631 ; Oughtred in 16333 Herigone in 1634 5 Sal- 
tonftall in 1636 ; Norwood in 1637 ; Fournier in 1643 ; 
and by almoft all the lucceeding writers on navigation of 
every country. 
To heave the log, as they call it, they throw it into the 
water on the lee fide, letting it run till it comes without 
the eddy of the (hip’s wake; then one, holding a half¬ 
minute glafs, turns it up juft as the firft knot, or the mark 
from which the knots begin to be reckoned, turns off the 
reel, or pafles over the ftern. As focn as the glafs is out, 
the reel is (topped, and the knots run off are told, and 
their parts estimated. It is ufual to heave the log once 
every hour in (hips of war and Eaft Indiarnen, ancT in all 
VOL. XII. No. 879. 
other vefiels once in two hours, allowance being made for 
the wind having increafed or abated in the intervals. 
The log is a very precarious way of computing, and 
mull always be corrected fisy experience, there being much 
uncertainty, from the motions of the (hip, the winds of 
variable force, the fridion of the reel, and lightnefs of the 
log in the courfe of the current. Yet this is a much more 
exaCt way of computing than any other in ufe; much 
preferable certainly to that of the Spaniards and Portu- 
guefe, who gueffed at the (hip’s way by the running of 
the froth or water by the (hip’s fide ; or to that of the 
Dutch, who ufe to heave a chip overboard, and to num¬ 
ber 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 fubjeCt to drive with the motion of the water 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, and many me¬ 
thods propofed to remove or to leflen them. M. Bouguer 
(Mem. Acad. Scienc. 1747.) propofed the following me¬ 
thod. Take for the log a conical piece of wood, which 
fix to the log-line, pafled through or along its axis, at 50, 
60, 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^ inches fquare; the interfeftion of the di¬ 
agonals 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 u(ed in Britifli dock-yards, when 
formed into a cone of the above dimerifions, will float a 
diver made of (tout tin plates, one fide of the fquare be¬ 
ing 9J inches. Such a diver, weighing iflb. avoirdupois, 
required to float it a cone of five inches diameter and 
twelve inches on the flant fide, fo as the point of the 
cone, which was made of light fir, (hould juft appear above 
the water. Now, fuppofing one fide of fuch a fquare tin 
diver to be about ten inches, and made of plates only two- 
thirds of the thicknefs of the former, fuch 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 bafe, 
and ten inches in the flant height or length ; and fuch a 
compound log might perhaps be found on trial to be af¬ 
fected by about as much again as that propofed by M. 
Bouguer; and confequently the difference between the 
numbers given by the common log and compound log, 
mult be augmented by two-thirds of itfelf for the necef- 
fary correction, as below. When the compound log of 
Bouguer, above deferibed, is hove overboard, the diver 
will fink too deep to be much afFeCted by the current or 
motion of water at the furface, and the log will thereby 
keep more fteadily in the place where it firlt fell; and 
confequently the knots run off the reel will (how more ac¬ 
curately the (hip’s rate of failing. As the common log is 
afFeCted by the whole motion of the current, fo this com¬ 
pound log will feel only a part thereof, viz. fuch a part 
nearly as the refiftance of the cone is to the refiftance of 
the diver; then the refiftances of the above cone and diver 
are about as 1 to 5 ; and confequently this log will drive 
but one-fifth part of what the common log would do; 
and fo the (hip’s true run will be affeCted by one-fifth 
only of the motion of the W'ater. To obtain the true 
rate of failing, 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 fourth part, is the correClion; which applied to the 
knots of the common log, will give the (hip’s true rate of 
failing at the middle time between the hours when thefe 
logs were hove. The correction is additive when the 
compound log’s run is the greateft, otherwifs it is fub- 
10 P tractive. 
