C;i ai\ VIII.
8 43
with sights, C, C, at its opposite ends, and two narrow slits &, c for observations; in themiddle of the bar is a circular brass box, A, containing a magnetic needle, and coveredwith glass. The ends of the needle play over a brass circle, g , which is divided into360 equal degrees, in such a manner that the two numbers of 90° are at right anglesto the lines drawn through the sights. This instrument is usually supported on astaff or tripod, E, and when firmly fixed in the ground it can he turned in any direction bymeans of its socket joint When the magnetic needle is well balanced, and movesfreely in its horizontal position, the sight can be turned towards the object to be sur-veyed, and the needle will retain its positionof due north and south; consequently thenumber of degrees which the angle contains,after moving from one object to another, canbe counted off. The length of the mag-netic needle increases the accuracy of thecircumferenter, for if made too small it willbe apt to follow the motion of the instrumentwhen turned round.
This instrument is chiefly used in minesand coalpits, and sometimes has a spiritlevel attached to it; a a are screws to ad-just and turn it round, which is performedby moving the vertical screws at in ; F is aspirit level; but by the free play of the needleit may be generally ascertained whetherthe circumferenter is nearly or perfectly level.
When this is the case, by clamping theball and socket joint tight, and turning thesights to the respective objects, the mea-surement in degrees of the angle made may berelied upon. The needle should not be suf-fered to play longer than is necessary, butbe lifted off its centre, otherwise the delicatepoint upon which it turns would soon bedestroyed. This instrument usually has theeast and west marked contrary to their truepositions, in order that by the reading oil’tlie needle, the actual direction of the line isshown.
The Surveying Cross , employed for establishing perpendicular lines, is a better arrange-ment than the cross staff, being more carefully made; it consists of four sights, fixed atright angles upon a brasscross, which can be screwedto a tripod or single staff;by placing it with referenceto a given line, perpendi-culars can easily be traced onthe ground or set out.
A Circular Box of brass,called the optical square ,is a more convenient in-strument for the same pur-pose ; this contains the twoprincipal glasses of the sex-tant, viz. the index and ho-rizon glasses, fixed at an angleof 45° ; so that viewing anobject by direct vision, anyother forming a right anglewith it at the place of theobserver will be seen by reflection to coincide with the object viewed. Placing the in-strument in such a position as to look through any given line, we are enabled to directa station staff to be placed perpendicular to the given line. There are several varietiesof this instrument, some of which are fitted for the pocket, and extremely useful to the sur-veyor.
Prismatic Compass is a similar instrument, but differs from the circumferenter, byhaving a floating card attached to the needle graduated to 15' of a degree; but angles'cannot be taken with it of less than half a degree : these graduations commence at thenorth point, and are numbered 5°, 10°, 20°, &e. round the circle up to 360°. A sight
Fig. 1067.