THEODOLITE

, an instrument much used in surveying, for taking angles, distances, altitudes, &c.

This instrument is variously made; different persons having their several ways of contriving it, each attempting to make it more simple and portable, more accurate and expeditious, than others. It usually consists of a brass circle, about a foot diameter, cut in form of fig. 5, pl. 31; having its limb divided into 360 degrees, and each degree subdivided either diagonally, or otherwise, into minutes. Underneath, at cc, are fixed two little pillars bb (fig. 6), which support an axis, bearing a telescope, for viewing remote objects.

On the centre of the circle moves the index C, which is a circular plate, having a compass in the middle, the meridian line of which answers to the fiducial line aa; at bb are fixed two pillars to support an axis, bearing a telescope like the former, whose line of collimation answers to the fiducial line aa. At each end of either telescope is, or may be, fixed a plain sight, for the viewing of nearer objects.

The ends of the index aa are cut circularly, to sit the divisions of the limb B; and when that limb is diagonally divided, the fiducial line at one end of the index shews the degrees and minutes upon the limb. It is also furnished with cross spirit levels, for setting the plane of the circle truly horizontal; and a vertical arch, divided into degrees, for taking angles of elevation and depression. The whole instrument is mounted with a ball and socket, upon a three-legged staff.

Many Theodolites however have no telescopes, but only four plain sights, two of them fastened on the limb, and two on the ends of the index. Two different ones, mounted on their stand, are represented in fig. 2 and 3, plate 33.

The use of the Theodolite is abundantly shewn in that of the semicircle, which is only half a Theodolite. And the index and compass of the Theodolite serve also for a circumferentor, and are used as such.

The ingenious Mr. Ramsden has lately made a most excellent Theodolite, for the use of the military survey now carrying on in England.