Clockmaker's COMPASSES are joined like the common
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 paste-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 ellipsis; 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.