A mathematical instrument used by surveyors for taking angles by the magnetic needle in cases where great accuracy is not required, and where a permanent direction of the needle is of the most material consequence in surveying. It consists of a compass box containing a magnetic needle, supported on a pivot in the centre, and the circumference of the box divided into 360 parts or degrees. The box is furnished with two sights, placed on opposite ends of the meridian line, or 180° asunder; it is also mounted on a pivot in the head of a tripod or stand. In taking the angle between two objects with this instrument, the box is turned until one object is seen through the sights, when the number of degrees to which the south end of the needle points is noted; the box is then turned until the second object be observed through the sights, and the degrees pointed to by the needle again noted, and the difference between the two numbers is the quantity of the angle.