Compass, Inclination.
A magnetic needle mounted on a horizontal axis at its centre of gravity,
so as to be free to assume the dip, or magnetic inclination, when placed
in the magnetic meridian. It moves over the face of a vertical graduated
circle, and the frame also carries a spirit level and graduated
horizontal circle. In use the frame is turned until the needle is
vertical. Then the axis of suspension of the needle is in the magnetic
meridian. The vertical circle is then turned through 90° of the horizon,
which brings the plane of rotation of the needle into the magnetic
meridian, when it assumes the inclination of the place.