SEMICIRCLE

, in Geometry, is half a circle, or a figure comprehended between the diameter of a circle, and half the circumference.

Semicircle is also an instrument in Surveying, sometimes called the graphometer.

It consists of a semicircular limb or arch, as FIG (fig. 3, pl. 26) divided into 180 degrees, and sometimes subdivided diagonally or otherwise into minutes. This limb is subtended by a diameter FG, having two sights erected at its extremities. In the centre of the Semicircle, or the middle of the diameter, is fixed a box and needle; and on the same centre is fitted an alidade, or moveable index, carrying two other sights, as H, I: the whole being mounted on a staff, with a ball and socket &c.

Hence it appears, that the Semicircle is nothing but half a theodolite; with this only difference, that whereas the limb of the theodolite, being an entire circle, takes in all the 360° successively; while in the Semicircle the degrees only going from 1 to 180, it is usual to have the remaining 180°, or those from 180° to 360°, graduated in another line on the limb within the former.

To take an Angle with a Semicircle.—Place the instrument in such manner, as that the radius CG may hang over one leg of the angle to be measured, with the centre C over the vertex of the same. The first is done by looking through the sights F and G, at the extremities of the diameter, to a mark fixed up in one extremity of the leg; and the latter is had by letting fall a plummet from the centre of the instrument. This done, turn the moveable index HI on its centre towards the other leg of the angle, till through the sights fixed in it, you see a mark in the extremity of the leg. Then the degree which the index cuts on the limb, is the quantity or measure of the angle.

Other uses are the same as in the theodolite.