COMPOS S. 
compafs, or winds, fixed on a magnetic needle that always 
turns to the north, excepting only a fmall deviation. The 
needle with the card turns on an upright pin fixed in the 
center of the box. To the middle of the needle is fixed 
a brafs conical focket or cap, by which the card hanging 
on the pin turns freely round the center. The top of the 
box is covered with a glafs, to prevent the wind from 
difturbing the motion of the card. The whole is inclofed 
in another box of wood, where it is fufpended by brafs 
hoops or gimbals, to keep the card in a horizontal pofition 
during the motions of the fiiip. The whole mull be fo 
placed in the fiiip, that the middle fe£tion of the box, 
parallel to its fides, may be parallel to the middle fedlion 
of the fiiip along its keel. 
The invention of the compafs was an ineftimable pre- 
fent to the mariner, by giving him a fure direction through 
the pathlefs ocean. It is ufually al'cribed to Flavio Gioia, 
or Flavio of Malphi, about the year 130?.; and hence it 
is that the territory of Principato, the part of the king¬ 
dom of Naples where he was born, has a compafs for its 
arms. Fie divided his compafs only into eight points. 
Others aferibe the invention to the Cliinefe ; and Gilbert, 
in his book de Mngnete , affirms that Marco Paolo, a Ve¬ 
netian, making a journey to China, brought back the in¬ 
vention with him in 1260. What flrengthens this conjec¬ 
ture is, that at firlt they ufed the compafs, in the fame 
manner as the Chinefe itill do, viz. letting it float on a 
final! piece of cork, inftead of fufpending ic on a pivot. It 
is added, that their emperor Chiningus, a celebrated af- 
trologer, had a knowledge of it jiso years before Chrift. 
Bur Ludi Vertomanus affirms, that when he was in the 
Eaft Indies, about the year 1500, he favv a pilot of a fiiip 
.direft his courfe by a compafs, faftened and framed as 
thofe now commonly ufed. And Barlow, in his book 
called the Navigator’s Supply, anno 1597, lays, that in a 
perfonal conference with two Eaft-Indians, they affirmed, 
that inftead of our compafs, they ufe a magnetical needle 
of fix inches, and longer, upon a pin, in a difii of white 
.earth filled with water; in the bottom of which they 
have two crofs lines fot the four principal winds, the relt 
of the divifions being left to the (kill of their pilots. Alfo 
in the fame book he lays that the Portuguefe, in their firfi 
difeovery of the Eaft Indies, got a pilot of Mahinde, who 
brought them from thence in thirty-three days, within 
fight of Calicut. But Faucliette relates fome verfes of 
Guoyot de Provence, who lived in France about the year 
1200, which feem to make mention of the compafs under 
the name of marinette, or mariner's jlone ; which ffiew it was 
ufed in France near one hundred years before either the 
Malphite or Venetian one. The French even lay claim to 
the invention, from the fleur-de-lys with which molt peo¬ 
ple diftinguifli the north point of the card. With as much 
reafon Dr. Wallis aferibes it to the Englifh, from its name 
compafs, by which name molt nations call it, and which, 
he obferves, is ufed in many parts of England to fignify 
a circle. 
The mariner’s compafs was long very rude and imper- 
fe£t, but at length received great improvement from the 
invention and experiments of Dr.Knight, who difeovered 
the ufeful practice of making artificial magnets; and whole 
fleering compafs is now in ufe in all our Ihips of war. 
The needle in this inftrument is quite ftraight, and fquare 
at the ends; and confequently has only two poles, though 
about the hole in the middle the curves are fomewhat 
confufed. Needles of this conftru<Slion, after vibrating a 
long time, will always point exadlly in the fame direc¬ 
tion ; and if drawn ever fo little on one fide, will return 
to it again, without any fenfible difference. We may 
therefore conclude, that a regular parallelopiped is the 
bell form for a needle, as well as the fimplell, the holes 
for the caps being as fmall as poffible. And, as the weight 
fliould be removed to the greatell dillance from the cen¬ 
ter of motion, a circle of brafs, of the fame diameter of 
the card, may be added, which will lerve alfo to fupport 
tfce card, which may then be made of thin paper, without 
837 
any thing fo ftift'en if. This ring being fixed below the 
card, and the needle above it, tire center of gravity is 
placed iow enough to admit of the cap being put under 
the needle, whereby the hole in the needle becomes uti- 
neceflary. Thefe oblervations will be eafily underftood, 
from viewing the feveral parts of the inftrument, as deli¬ 
neated in the annexed engraving, where fig. 1. reprelents 
the card, with the needle, and its cap fixed upon it. 
Fig.-2. is a perfpeflive view of the back-fide o,f the card, 
where AB reprelents the turning down of the brafs edge, 
C the under part of the cap, D and E two Hiding weights 
to balance the card, and F,G, two ferews that fix the 
brafs edge, &c. to the needle. Fig. 3. is the pedeftal that; 
Supports tiie card, containing a ferevving needle, fixed in 
two fmall grooves to receive it, by means of the collet C, 
in the manner of a port-crayon ; D, the Hem, filed into 
an odlagon, that it may be the more eafily unlcrewed. 
For its further illuflration and application to ule, fee 
Navigation. 
The compafs is fometimes difturbed by the eleflricity 
of its glafs cover; the. remedy for this inconvenience is 
to moiften the fur face of the glafs with -a wet finger, 
which removes it immediately and effeftually. The ma¬ 
riner’s compafs with a chart is much lefs dangeroufiy 
moved than the common compafs with a bare needle; 
and the deeper, or farther diftant the needle hangs below 
the glafs, the lefs dilturbance it is likely to receive. But, 
notwithstanding the various contrivances that have been 
made to prevent the card from being much affected by 
the motions of the Ihip, they have always been found too 
delicate to encounter the (hocks of a tempelluous fea. 
This defeat, however, has been lately in a great meafure 
overcome, by a compafs conftrudled by Mr. M'Culloch, 
of London. We have given a reprefentation ol it in the 
plate, where fig. 4. is a feition of this fleering compafs: 
aa, the common wooden-box, with its lid ; bb, the brafs 
compafs-box; cc, the glafs cover to ditto ; riV, the hollow 
conical bottom ; ee, the prop upon which the compafs is 
fupported inftead of gimbals; the fpherical top of which 
is finely polilhed, and the apex of the hollow cone fitted 
in a peculiar manner to receive it; ff, a quantity of lead 
run round the bottom and cone of the compafs-box, to 
balance and keep it fteadily horizontal; gg, the card and 
the magnetical needle, bent in fuch a manner that the 
point of the conical pivot on which it moves and is fup¬ 
ported, may b^brought very near to the center of gra¬ 
vity, as well as to the center of motion; bb, two guards, 
which, by means of two pins, ii, affixed to the compafs- 
box, prevent it from turning round and deceiving the 
fteerfman. Fig. 5. is a perfpedtive view of this (leering 
compafs, with the lid off and the front laid open ; bh, the 
guards ; b, the compals-box ; e, the prop. See. 
The azimuth compafs differs from the common fea 
compafs in this; that there is faftened, on the round box' 
wherein the card is, a broad circle AB, fig. 6. one-half 
whereof is divided into ninety degrees, and thole lubdi- 
vided diagonally into minutes; b is a moveable index, 
having a fight, a, erefted thereon, and moving on a hinge. 
From the upper part of the fight to the middle of the in¬ 
dex, is faftened a fine hypothenufal luteftring, to give a 
lhadow on the line in the middle of the index. The cir¬ 
cle AB is croffed at right angles with two threads, from 
the' extremities whereof are drawn four lines on the in- 
fide of the round box; there are alfo four lines drawn at 
right angles to each other on the card. The round box 
fitted with its card, graduated circle, and index,, is hung 
in the brafs hoops BB, and thefe hoops faftqned fo the 
fquare box CC. An improvement on the azimuth com¬ 
pafs has likewife been made by Mr. M'Culloch, as repre¬ 
sented at fig. 7. in which b Ihews the compals-box ; h, one 
of the guards; e, the prop, as in fig. 6. with this diffe¬ 
rence, that, inftead of being ferewed to the.bottpm of the 
wood-box, it Hands in a brafs focket, and may be turned 
round at pleafure; 1, a brafs bar, upon which the fighf- 
vaues are fixed $ 2, a dark glafs, which moves up or down 
01 * 
