COMPAQ S. 
Other parts of the world, a variation chart may be at¬ 
tended with confiderable benefit. Hotvever, the variation 
curves, when they run eaft and weft, may fometimes be 
applied to good purpofe in correcting the latitude, when 
meridian obfervations cannot be had, as it often happens 
on the northern coafts of America, in the Weftern Ocean, 
and about Newfoundland; for if the variation can be ob¬ 
tained exactly, then the eaft and weft curve, anfwering to 
the variation in the chart, will fhew the latitude. Se¬ 
condly, As the deviation of the magrietical meridian, 
from the true one, is fubjedl to continual alteration, there¬ 
fore a chart to which the variation lines are fitted for any 
year, muft in time become ufelefs, unlefs new lines, fhew- 
ing the lfate of the variation at that time, be drawn on 
the chart; but as the change in the variation is very flow, 
new variation charts publilhed every feven or eight years 
will anfwer the purpofe tolerably well. And thus it has 
happened that Halley's variation chart has become ufe- 
3 efs, for want of encouragement to renew it from time to 
time. However, in 1744, Mr. William Mountaine, and 
Mr. James Dodfon, publilhed a new variation chart, 
adapted for that year, which was well received; and fe- 
veral inftances of its great utility having been communi¬ 
cated to them, they fitted the variation lines anew for the 
year 1756, and in the following year publilhed the third 
variation chart, and alfo p re fen ted to the royal fociety a 
curious paper concerning the variation of the compafs, 
with a fet of tables annexed, containing the refult of up¬ 
wards of fifty thoufand obfervations, in fix periodical re¬ 
views, from the year 1700 to 1756 inclufive, and adapted 
to every five degrees of latitude and longitude in the more 
frequented oceans 5 which paper and tables were printed 
in the Tranfaftions for 1757. 
From thefe tables of obfervations, fuch extraordinary 
and whimfical irregularities occur in the variation of the 
compafs, that we cannot think it wholly under the di¬ 
rection of one general and uniform law ; but rather con¬ 
clude, with Dr. Gowen, in the eighty-feventh propofition 
of his Treatife upon Attraction-and Repulfion, that it is 
influenced by various and different magnetic attractions, 
perhaps occafioned by the heterogenous compofitions in 
our great magnet, the earth. This doCtrine of the varia¬ 
tion will alfo apply to the Compass Dial, which is a 
finall dial fitted for the pocket, to fhew the hour of the 
day, by the direction of the needle that indicates how to 
place them right, by turning the dial about till the cock 
or ftyle ftand direCtly over the needle. But thefe can 
never be very exaCt, becaufe of the variation of the needle 
itfelf; unlefs that variation be allowed for, in making 
and placing the inftrument. 
Of the COMMON COMPASSES. 
The common compafles confift of two fharp-pointed 
branches or legs of iron, lteel, brafs, or other metal, 
joined together at the top by a rivet, about which they 
move as on a centre. Thofe compafles are of the belt fort 
in which the pin or axle, on which the joint turns, is 
made of fteel, and alfo half the joint itfelf, as the oppo- 
fite metals wear more equally; the points Ihould alfo he 
made of hard fteel, well polifhed; and the joint fhould 
open and fhut with a finootb, eafy, and uniform, motion. 
In feme compafles, the points are both fixed; but in 
others, cne is made to take out occafionally, and a draw¬ 
ing-pen, or pencil, put in its place. There are in ufe 
compafles of various kinds and contrivances, adapted to 
the various purpofes they are intended for; as, 
Compares of three Legs, or : Triangular Compaffes 5 the 
conftruCtion of which is like that of the common com- 
paffes, with the addition of a third leg or point, which 
has a motion every way. Their ufe is to take three points 
at once, and fo to form triangles, and lay down three 
pofitions of a map to be copied at once. 
Beam Compares confift of a long ftraigbt beam or bar, 
carrying two brafs curfors ; one of thefe being fixed at 
©pe end, the other Aiding along the beam, with a ferew 
' Vo l. IV. No. 247, 
JI 93 
to fatten it on occafionally. To the curfors may be ferew- 
ed points of any kind, whether fteel, pencils, or the like. 
To the fixed curfor is fometimes applied an adjulting or 
micrometer ferew, by which an extent is obtained to very 
great nicety. The beam compafles are ufed to draw large 
circles, to take great extents, &c. 
Bo-zu Compaffes, or Bozos, are a finall fort of compafles, 
that fhut up in a hoop, which ferves for a handle. Their 
ufe is todeferibe arcs or circumferences with a very finall 
radius. 
Caliber Compaffes. See Callipers, vol.iii. p. 628. 
Clcckmakers Compaffes are jointed like the common com- 
pafles, with a quadrant or bow like the fpring compafles; 
only of different ufe, ferving here to keep the inftrument 
firm at any opening. They are made very ftrong, with 
rhe points of their legs of well-tempered fteel, as being 
ufed to draw ©r cut lines in pafteboard, copper, &c. 
Cylindrical and SpkericalCompafes , ccnfiftof fourbranches 
joined in a centre, two of them being circular and two 
flat, a little bent at the ends. The ule of them is to take 
the diameter, thicknefs, or.caliber, of round or cylindrical 
bodies; as cannons, halls, pipes, &c. There are alfo 
fpherical compafles, differing in nothing from the com¬ 
mon ones, but that their legs are arched ; ferving to take 
the diameters of round bodies. There is alfo another 
fort of compafles lately invented, for meafuring the dia¬ 
meter of round bodies, as balls, Sec. which confift of two 
flat pieces of metal fet at right angles on a ftraight bar or 
beam of the fame; the one piece being fixed, and the 
other Aiding along it, fo far as juft to receive the round 
body between them; and then its diameter, or diftance 
between the two pieces, is fhewn by the divilions marked 
on the beam. 
Elliptical Compaffes, are ufed to draw ellipfesor ovals of 
any kind. The inftrument conlifts of a beam about a 
foot long, bearing three curfors ; to one of which may be 
ferewed points of any kind ; and to the bottom of the 
other two are rivetted two Aiding dove-tails, adjufted in 
grooves made in the crofs branches of the beam. The 
dove-tails having a motion every way, by turning about 
the long branch, they go backward and forward along 
the crofs; fo that when the beam has gone half way 
round, one of thefe will have moved the whole length of 
one of the branches; and when the beam has gone quite 
round, the fame dove-tail has gone back the whole length 
of the branch. The diftance between the two Aiding 
dove-tails, is the diftance between the two foci of the el- 
lipfe; fo that by changing that diftance, the ellipfe will 
be rounder or flatter. Under the ends of the branches of 
the crofs, are placed four fteel points tp keepitfaft. The 
ufe of this compafs is eafy : by lurninground the long 
branch, the pen, pencil, or other points, will draw the 
ellipfe required. 
German Compaffes, have their legs a little bent out¬ 
wards, near the top; fo that when fhut, the points only 
meet. 
Hair Compaffes, are fo contrived within fide by a fmall 
adjulting ferew to one of the legs, as to take an extent t» 
a hair’s breadth, or great exaftnefs. 
Proportional Compaffes, are thofe whofe joint lies, not 
at the end of the legs, but between the points terminat¬ 
ing each leg. Thefe are either Ample, or compound. In 
the former fort the centre, or place of the joint is 
fixed ; fo that one pair of thefe ferves only for one pro¬ 
portion. 
Compound Proportional Compaffes, have the joint or cen¬ 
tre moveable. They conlilt of two parts or fides of 
brafs, which lie upon each other fo nicely as to feem but 
one when they are fhut. Thefe fides eafily open, and 
move about the centre, which is itfelf moveable in a hol¬ 
low canal cut through the greateft part of their length. 
To this centre on each fide is fixed a Aiding piece, of a 
fmall length, with a fine line drawn on it ferving as an in¬ 
dex, to be let againft other lines ordivifions placed upon 
the compafles on both fides. Thefe lines are, 1, a line of 
10$ lines; 
