an instrument used by surveyors for taking angles.

It consists of a brass index and circle, all of a piece. The index is commonly about 14 inches long, and an inch and a half broad; the diameter of the circle is about seven inches. On this circle is made a chart, whose meridian line answers to the middle of the breadth of the index, and is divided into 360 degrees. There is a brass ring soldered on the circumference of the circle, on which screws another ring, with a flat glass in it, so as to form a kind of box for the needle, suspended on the pivot in the centre of the circle. There are also two sights to screw on, and slide up and down the index, as also a spangle and socket screwed on the back side of the circle for putting the head of the staff in.

How to observe the Quantity of an angle by the Circumferentor. Let it be required to find the quantity of the angle EKG; first place your instrument at K, with the flower-de-luce of the chart towards you; then direct your sights to E, and observe what degrees are cut by the fourth end of the needle, which let be 296; then, turning the instrument about, direct your sights to G, noting then also what degrees are cut by the fourth end of the needle, which suppose 247. This done, always subtract the lesser from the greater, as in this example, 247 from 296, the remainder is 49 degrees, which is the true quantity of the angle ERG.

A circumferentor was made by Mr Jones of Holborn on an improved construction. From a very simple contrivance, it is rendered sufficient to take angles with the accuracy of a common theodolite; and by its angles of altitude and depression may be observed as readily as horizontal ones. The improvement chiefly consists in an arm or index (G), so applied to the centre of the compass box, and within it, that, at the time of observing, by only flipping a pin (p) out, the circle of degrees alone may move round, and leave the index (G) fixed. This index will remain stationary, from its being attached to the socket that screws on the head of the staffs. On the end of this index, next the degrees in the box, there is graduated a nonius scale, by which the circle of 360 degrees is subdivided into five minutes or less if desired. To take angles of altitudes or depressions, the instrument is turned down on its ball and socket into a perpendicular position, and adjusted to its level by a plumb line (O), that is hung on a pin at the back of the box, and made to coincide with a mark made thereon. Then by looking through the small sight holes (x) purposely made, the angles are shown on the circles of degrees by the nonius as before. The arms (AA.) of the instrument flip off (at BB.), and the whole packs into a case but 5½ inches square and 3 deep.