Pair of Compasses, a mathematical instrument for describing circles, measuring figures, &c.

The common compasses consist of two sharp-pointed branches or legs of iron, steel, brass, or other metal, joined together at the top by a rivet, whereon they move as on a centre.

The principal perfection of this, as of all other compasses, consists in the easy and uniform opening and shutting of their legs; one of which may be taken out, in order to make room for others.

There are now used compasses of various kinds and contrivances, accommodated to the various uses they are intended for; as,

Compasses of three Legs are, setting aside the excess of a leg, of the same structure with the common ones: their use being to take three points at once, and so to form triangles; to lay down three positions of a map, to be copied at once, &c.

Beam Compasses consist of a long branch, or beam, carrying two brass curlicues, the one fixed at one end, the other sliding along the beam, with a screw to fasten it on occasion. To the curlicues may be screwed points of any kind, whether steel, for pencils, or the like. It is used to draw large circles, to take great extents, &c.

Caliber Compasses. See Caliber.

Clockmaker's Compasses are joined like the com- Compasses, mon compasses, with a quadrant, or bow, like the spring compasses; only of different use, serving here to keep the instrument firm at any opening. They are made very strong, with the points of their legs of well tempered steel, as being used to draw lines on pastle-board or copper.

Cylindrical and Spherical Compasses, consist of four branches, joined in a centre, two of which are circular, and two flat, a little bent on the ends: their use is to take the diameter, thickness, or caliber of round or cylindric bodies; such as cannons, pipes, &c.

Elliptic Compasses consist of a cross ABGH, (plate LXXXIV. fig. 9.) with grooves in it, and an index CE, which is fastened to the cross by means of dove-tails at the points CD, that slide in the grooves; so that when the index is turned about, the end E will describe an ellipse; which is the use of these compasses.

German Compasses have their legs a little bent outwards, towards the top; so that when shut, the points only meet.

Lapidary's Compasses are a piece of wood, in form of the shaft of a plane, cleft at top, as far as half its length; with this they measure the angles, &c. of jewels and precious stones, as they cut them. There is in the cleft a little brass rule, fastened there at one end by a pin; but so that it may be moved in the manner of a brass level: with this kind of square they take the angles of the stones, laying them on the shaft as they cut them.

Proportional Compasses are such as have two legs, but four points, which, when opened, are like a cross, as not having the joint at the end of the legs like common compasses: some of these have fixed joints, others moveable ones; upon the legs of the latter of which are drawn the lines of chords, sines, tangents, &c. Their use is to divide lines and circles into equal parts; or to perform the operations of the sector at one opening of them.

Spring-Compasses, or dividers; those with an arched head, which by its spring opens the legs; the opening being directed by a circular screw fastened to one of the legs, and let through the other, worked with a nut. These compasses are made of hardened steel.

Trifolding Compasses consist of two central rules, and an arch of a circle of 120 degrees, immovable, with its radius; which is fastened with one of the central rules like the two legs of a sector, that the central rule may be carried through all the points of the circumference of the arch. The radius and rule should be as thin as possible; and the rule fastened to the radius should be hammered cold, to attain the greater elasticity; and the breadth of the central rule should be triple that of the radius: there must also be a groove in this rule, with a dove-tail fastened on it for its motion, and a hole in the centre of each rule. The use of this instrument is to facilitate the trisection of angles geometrically; and it is said to have been invented by M. Tarragen for that purpose.

Turn-up Compasses. The body of this instrument is like the common compasses: but towards the bottom of the legs, without-side, are added two other points besides the usual ones; the one whereof carry a drawing pen point, and the other a port-crayon, both adjusted so as to turn round, and be in the way of use, or out of it, as occasion requires. These compasses have been contrived to save the trouble of changing the points.