620 
N A V I G A T I O N. 
the fliip ; and, for the moft ready difcovery of this point 
of commencement, there is commonly faftened to it a 
piece of red rag; that part of the line between the red rag, 
and the log is called the firay-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 eafy, and about which 
it is wound) as fall as the log will carry it away, or 
rather as fall as the fliip fails from it, will fhow how fall 
the fliip has failed in the given time, or rate of failing per 
hour. 
The experiment for finding the velocity of the fliip is 
called heaving the log. 
Care fliould be taken to veer out the line as fall 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 reckon¬ 
ing. 
In the king’s fliips, India (hips, and fome others, the 
log is hove every hour; but in coafters, and thofe ufing 
fhort voyages, every two hours. Here the fliip 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 has been more fail fet, or any handed, that fo 
the fliip has run more or lefs in any part of the hour than 
file did at the time of the experiment; or if it fliould fall 
lefs or more wind at that time; there mull be allowance 
made for it according to the dilcretion of the officer. 
Sometimes, too, when the fliip is before the wind, and a 
great lea letting after her, it will bring home the log; in 
fuch cafes it is cuftomary to allow one mile in ten, and 
lefs in proportion, if the lea be not fo great. 
Care fliould alfo be taken to meafure the log-line pretty 
often, left it ftretch, and deceive you in the diftance. The 
like regard mull be had that the half-minute glafs be 
juft 3o"feconds, otherwife no account of the Ihip’s-way 
can be kept; to prove which, if there be no Hop-watch at 
hand, let a plummet, of any form or weight, be faftened 
to a filk firing or thread, with a loop to hang on a fmall 
pin or nail faftened in any place, fo that the plummet 
may fwing freely ; let it be 39I 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 pafies the per¬ 
pendicular let fall from the pin, and every time it pafies 
from the perpendicular to the utmoft fwing will be half 
a fecond. 
How to correct the Dijlance given by the Log-Line and Half- 
Miniite GlaJ's. 
The diftance 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: 
Cafe 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. 
Ex. 1. Suppofe a fliip runs at the rate of knots in 
the time the glafs runs out, but meafuring the glafs I find 
it runs 34. feconds ; what is the true rate of failing ?—As 
34 : 30 :: 7'S ‘. 6-6 miles, the true diftance failed in an 
hour. 
Ex. 2. Suppofe a fliip runs at the rate of knots, but 
meafuring the glafs I find it runs only 25 leconds ; re¬ 
quired the true rate of failing ?—As 25 ; 30 ;; 6-5 ; 7-8 
miles, the true diftance failed in an hour. 
II. When the glafs is true, and log-line faulty. Say, 
As 50 feet is to the diftance meafured between knot and 
knot, fo is the diftance run by the log to the true dif¬ 
tance. 
Ex. 1. Suppofe a ffiip runs at the rate of 6J knots in 
half a minute, but, meafuring the fpace 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. 
Ex. 2. Suppofe a fliip runs at the rate of 6\ knots in 
halt a minute, but, meafuring the fpace between knot 
and knot,. I find it to be only 44. teet; required the true 
rate ot failing.-—As 50 : 44 ;; A5 ; 572 miles, the true 
diftance failed in an hour. 
HI- When both the log-line and glafs are faulty. 
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 glafs will give the true diftance 
run. 
Ex. Suppofe a (hip runs 5 knots of a log-line of 45 feet 
to a knot, wdiile a glafs ot 25 feconds is running out; what 
is the true rate of failing ? 
The meafured length of a knot - - 45 
Multiplied by - - - - - - 3 
Gives thrice the meafured length of a knot 135 
Which multiplied by the diftance run per log 5 
ProduCl 675 
And, dividing the produCl by 
5 times the time the glafs runs, 
that is, 5X2-5 = 125, the quo¬ 
tient is 5'4, the number of miles 
the fliip runs per hour. 
This rule is only a compound 
of the two former Ample ones, 
which is contracted a little. 
When the glafs is faulty, the 
log-line may be divided as in 
the annexed Table, ftiowing the 
length of the knots of the log¬ 
line of different glafl’es. 
Of the Variation of the Compass. 
The variation of the compafs is an arch of the horizon 
contained between the meridian of the place and the mag¬ 
netic meridian, and is either eaft or weft; or it is the num¬ 
ber of degrees, &c. the needle’s point Hands from the true 
north or fouth points of the horizon, reckoned to the 
eaftward or weftward, and is readily found from the fun’? 
amplitude or azimuth. 
To find the true Amplitude. 
The fun’s true amplitude is an arch of the horizon, 
comprehended between the true eaft or weft points thereof, 
and the centre of the fun at its riling or fetting; or it is 
the number of degrees, &c. the fun rifes or fets to the 
northward or fouthward of the eaft or weft point of the, 
horizon. 
The fun’s magnetic amplitude is the number of degrees. 
See. the centre is northward or fouthward of the eaft or 
weft points of the compafs at his rifing or fetting, and is 
found with an azimuth-compafs in the following manner .- 
Having placed the azimuth-compafs in a convenient part 
of the fliip, look direCtly through the fight-vanes at the 
fun’s centre ; and, when the fun’s lower edge juft touches 
the horizon, flop the card, by a Hop which is placed on 
the compafs for that purpofe ; then the quantity of de¬ 
grees and minutes contained between, the eaft or weft, 
and the north or fouth, points of the compafs, will be the 
magnetic amplitude. 
The true amplitude is found either by infpeClion in the 
Tables of the Sun’s Amplitude, or by calculation as fol¬ 
lows : 
Second 
of Glafs. 
Length of 
Knots in Feet. 
2 V 
40*0 
25 
41*8 
26 
43’4 
27 
45-0 
28 
46-8 
29 
48-4 
30 
50; 0 
31 
5 i -8 
32 
53‘4 
33 
55-0 
34 
56-8 
35 
58-4 
36 
6o'o 
Rule, 
