AZIMUTH COMPASS.
Nothing is easier. An azimuth compass is all that is necessary for all but the most accurate inquiries.
SECTION OF AZIMUTH COMPASS.
A, needle and card; P, prism; SV, directrix or frame carrying a wire directed to the object and seen over the prism while the prism reflects to the eye the division of the scale underneath it.
The azimuth compass is an instrument familiar to many; it consists of a magnetic needle fastened to a card carrying a circle divided into 360°, which can be conveniently read by a prism when the instrument is turned toward any definite direction marked by a vertical wire. Its use depends upon the fact that at the same place and at the same time all magnetic needles point in the same direction, and the variation for the true north and south direction is either supposed to be known or can be found by observation.
THEODOLITE FOR DETERMINING AZIMUTH AND ALTITUDES.
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.