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A theodolite armed with a delicately hung magnetic needle, which can be rotated on a vertical axis, will do still better; it has first of all to be levelled. There is a little telescope with which we can see along the line. When we wish, for instance, to observe the amplitude of a temple, the theodolite is set up on its tripod in such a position that we can look along a temple wall or line of columns, etc., by means of the telescope. We then get a magnetic reading of the direction after having unclamped the compass; this gives the angle made between the line and the magnetic north (or south), as in the azimuth compass.

What we really do by means of such an instrument is to determine the astronomical meridian by means of a magnetic meridian. Here some definitions will not be out of place.

The meridian (meridies = midday) of any place is the great circle of the heavens which passes through the zenith (the point overhead) at that place and the poles of the celestial sphere.

The meridian line at any place is the intersection of the plane of the meridian with the plane of the horizon at that place, or, in other words, it is the line joining the north and south points. If we have the proper instruments, we can determine the meridian line astronomically at any place by one of the following methods:—