CIRCUMFERENTOR, a mathemati cal instrument, used by land-surveyors for taking angles by the magnetic nee dle. It is an instrument ( where great accuracy is not desired) much used in surveying, in and about woodlands, com mons, harbours, seacoasts, in the work. ing of coal-mines, &c. &c. where a per manent direction of-tile needle is of the must material consequence in surveying The instrument is made of brass, and, in its most simple state, consists of the fol lowing parts; a brass compass box, about five inches diameter, or more ; on the plate of the box are engraved and letter ed the principal points of the compass, divided into four quarters of 90 degrees each, two of the quarters being figured from the south point, and terminated by 90 degrees at the east and west ; and the other two quarters from the north point, terminating also at the east and west : on the circumference of the plate is fixed a ring, divided into 360 degrees, number ed from 0 to 360; the observer may therefore take his angles as bearing from the north and south towards the east and west ; or, by that which is the most usual method, the whole circum ference of a circle of 360 degrees, com mencing from the north point : a magnet ic needle of the usual kind turns upon an iron point, fixed in the centre of the compass plate ; a stop and trigger wire is applied to the compass box, to throw the needle off its centre when not in use, in order to preserve the fineness of the centre point : a glass and brass spring ring covers the needle and closes the box ; to the under side of the compass box, at the N. and S. points, is connected
a bar about 15 inches long, from end to end, to each end of which is fixed a per pendicular brass sight about five inches long ; each sight containing a long slit or perforation, and a sight line, so that the observer may take his line of sight, or observation of the line upon the sta tion mark, at which end of the bar he pleases.