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This brings us at once to a practical point. It will be asked, How can such an inquiry be prosecuted? How can the amplitudes of the temples be determined?

AZIMUTH COMPASS.

Nothing is easier. An azimuth compass is all that is necessary for all but the most accurate inquiries.

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.