N A V I G 
it is faid, may make an angle of about five points with 
it. In all thefe cafes, however, the velocity of the (hip is 
greatly retarded; and that, not only on account of the 
obliquity of her motion, but by reafon of what is called 
her lee-way. This is occafioned by the yielding of the 
water on the lee fide of the drip, by which means the 
veflel acquires a compound motion, partly in the dirediion 
of the wind, and partly in that which is necefl'ary for at¬ 
taining the defired port. 
In many parts of the ocean there are currents, or 
places where the water, inftead of remaining at reft, 
runs with a very confiderable velocity for a great way 
in fome particular direction, and which will certainly 
carry the (hip greatly out of her courfe. This occafions 
an error of the fame nature with the lee-way ; and there¬ 
fore, whenever a current is perceived, its direction and 
velocity ought to be determined, and the proper allow¬ 
ances made. 
Another fource of error in reckoning the courfe of a 
(hip proceeds from the variation of the compafs. There 
are few parts of the world where the needle points exactly 
north ; and in thofe where the variation is known, it is 
fubjeft to very confiderable alterations. By thefe means 
the courfe of the (hip is miftaken ; for, as the failors have 
no other ftandard todireft them than the compafs, if the 
needle, inftead of pointing due north, (hould point north- 
eaft, a prodigious error would be occafioned during the 
courfe of the voyage, and the (hip would not come near 
the port to which (he was bound. To avoid errors of this 
kind, the only method is, to obferve the fun’s amplitude 
and azimuth as frequently as poftible, by which the vari- 
htion of the compafs will be perceived, and the proper 
allowances can then be made for errors in the courfe which 
this may have occafioned. 
The art of navigation depends upon aftronomical and 
mathematical principles. The places of the fun and fixed 
liars are deduced from obfervation and calculation, and 
arranged in tables, the ufe of which is abfolutely necef- 
fary in reducingobfervations taken at fea, for the purpofe 
of afcertaining the latitude and longitude of the (hip, and 
the variation of the compafs. The problems in the vari¬ 
ous failings are refolved either by trigonometrical calcu¬ 
lation, or by tables or rules formed by the afliftance of 
trigonometry. By mathematics the necefl'ary tables are 
conftrufted, and rules inveftigated for performing the 
more difficult parts of navigation. 
The great end and bufinefs of navigation being to in¬ 
draft the mariner how to conduft a (hip through the wide 
and pathlefs ocean to the remoteft parts of the world, the 
fafeft and (liorteft way, in pafl'ages navigable ; to accom- 
plifli this, then, are requifite—A perfeft knowledge of 
the figure and motion of the earth, and the various real 
and imaginary lines upon it, (o as to be able to afcertain 
the real diftance and iituation of places with refpeft to 
one another; with the ufe of the feveral inftruments em¬ 
ployed in meafuring the (hip’s way ; fuch as the log, half¬ 
minute glafs, quadrant, or fextant, to take the altitude of 
the fun and ftars ; compafs, to reprefent the ffenfible hori¬ 
zon ; and azimuth-compafs, to take the azimuth or am¬ 
plitude of the fun, in order to know the variation of the 
magnetic needle; maps and charts of the feas and lands, 
together with the depth of water, the times and fettings 
of the tides Upon the coalls he may have occafion to ap¬ 
proach ; a competent knowledge of currents; of the 
mould and trim of the (hip, and the fail (lie bears, that fo 
due allowance may be made for lee-way. By help of 
thefe, and (kill in the navigator, he may know at all 
times the place the (hip is in ; as alfo which way he mull 
(leer, and how far, to gain his intended port. 
Of the Mariner’s Compass. 
Ships at fea are direfted from one place to another by 
means of an inftrument called the mariner’s compafs, 
which is an artificial reprefentation of the horizon of every 
place, by the means of a circular piece of paper, called a 
A T I O N. <319 
card, divided like the horizon into degrees and points* 
which are called rhumbs. Now, the card being properly 
fixed to a piece of (leel, called the needle, that has been 
touched with a loadftone (w'hofe property is fuch as to 
caufe one end of the needle fo touched to point towards 
the north, when turning freely on fomething fupporting 
it), all the points of the card will be direfted towards the 
Corresponding points of the horizon. Hence it follows, 
that in every place the north point of the card (hows the 
pofition of the meridian of that place, and fome one 
rhumb or point of the card will coincide with, or be di¬ 
refted along, the track that makes any given ^.ngle with 
the meridian ; confequently, by the help of the card Oi 
compafs, a (hip’ may be kept in any propofed track or 
courfe. 
A rhumb-line, or point, is a right line drawn from the 
centre of the compafs to the horizon, and is named from 
that point of the horizon it falls in with. The courfe is 
the angle which any rhumb-line makes with the meridian, 
and is fometimes reckoned in degrees, and fometimes in 
points of the compafs ; fo that if a (hip fails upon the Se¬ 
cond rhumb, or N. N. E. the courfe is 22 degrees 30 mi¬ 
nutes : and fo for any other. See the article Compass. 
vol. iv. 
Of the Log-line and Half-minute Glass. 
The 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 fufticient to make it fwim upright in the 
water. (See Log, vol. xii.) To this log is faftened a long 
line of about 150 fathoms, called the log-line, which is 
divided into certain equal fpaces, 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 experi¬ 
ment. They are called hunts, becaufe at the end. of each 
of them there is apiece of twine.with knots in it, reeved 
between the ftrands of the line: thefe pieces of twine 
(how how many knots run out in half a minute, and con¬ 
fequently the (hip’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 Englifti feet; therefore, a nautical 
mile being ^ part of 367,200 feet, that is, 6120 feet,'and 
fince half a minute is T i 5 part of an hour, the length of 
the knot on the log-line ought to be the part of 6120 
feet, or 51 feet. (In the Requifite Tables, pubiifhed in 
1802, the fea-mile is accounted 6078 feet.) But as, for 
the mod part, the (hip’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 (hip than after it, 
therefore 30 feet may be taken as the proper length of 
each knot, and thefe knots fubdivided into ten fathoms, 
each of five feet, which is certainly the bed adapted for 
praftice, and will correfpond with all the tables and in¬ 
ftruments ufed in navigation, as they are decimally di¬ 
vided, and confequently the (hip’s run determined with 
greater eafe and certainty. But fome experienced com¬ 
manders find, that the allowing 50 feet to a knot generally 
makes the (hip a-head of the reckoning; and,°to avoid 
danger, moftly divide the log-line into knots of 7 or 
fathoms of 6 feet each, to correfpond with a glafs that 
runs 28 feconds. Others again divide the feconds the 
glafs runs by 4, and take the quotient for the diftance in 
fathoms between the knots. Certain it is that, whatever 
length the knots are, the moll convenient way is to divide 
them into tenths. 
In hot or dry weather the glafs runs out fader than in 
mold or rainy weather; therefore care (hould be taken 
to try; what number of feconds the glafs runs. 
The knots commonly begin to be counted at the dif¬ 
tance of 10, 12, or 15, fathoms from the log, according to 
the largenefs of the (hip, that fo the log may be out ofthe 
drip’s wake, when it is thrown overboard, before they 
begin to count, left the eddies fliould fuck the log aft er 
the 
