Plain-com′pass.
A simple form of the surveyor's instrument. It has a needle about six inches long, a graduated circle, main plate, levels and sights, and is placed upon the brass head of the Jacob-staff. |
| Plain-compass. |
The compass-circle is divided to half-degrees on its upper surface, the whole degree marks being also cut down on the inside circumference, and is figured from 0 to 90 on each side of the center or “line of zeros.” The sights, or standards, have fine slits cut through nearly their whole length, terminated at intervals by large circular apertures through which the object sighted upon is more readily found.
The right and left hand edges of the sights have respectively an eye-piece, and a series of divisions, by which angles of elevation and depression, for a range of about twenty degrees each way, can be taken with considerable accuracy. This arrangement is termed a tangent-scale, the divided edges of the north sight being tangents to segments of circles having their centers at the eye-pieces and their points of contact with the tangent lines at the zero divisions of the scale.
