This section is from the book "The Engineer's And Mechanic's Encyclopaedia", by Luke Hebert. Also available from Amazon: Engineer's And Mechanic's Encyclopaedia.

A mathematical instrument used by land-surveyors, for taking angles, distances, altitudes, etc. This instrument is variously made, and provided with subordinate apparatus, according to the price, or the requirements of the purchaser. We shall describe one of the most generally useful. This consists of two concentric horizontal circles, the inner of which has, at the ends of one of its diameters, two perpendicular columns, on which rests the horizontal axis of a small meridian telescope. The vernier of the inner circle is made fast to an arbitrary division line of the outer one, and both circles are moved, together with the telescope, until the object sought for appears in its field. The outer circle is now fixed, and the inner one is turned round, until the telescope strikes the second object, whose angular distance from the first is to be measured. The inner circle is now fastened to the outer, and by means of a micrometer screw, the thread of the telescope is brought exactly upon the object.

The arc which the vernier of the inner circle has described on the outer one measures the angle which the two objects make at the common centre of the two circles.

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