For finding the latitude, longitude, and variation, invented by James Hunter, member of the Glasgow Philosophical Society. The indicator consists of a stand, supporting a circular plate of polished brass, about 14 inches in diameter, representing the horizon, and marked and numbered accordingly with the proper divisions. This horizon is surmounted by a semicircular plate, as a meridian, set at right angles to the plane of the horizontal plate, properly divided, and furnished with an index attached to a nonius, indicating minutes. This meridian plate is cut out at the centre to allow room for a pivot, or hinge, for other parts of the indicator. On one side of this meridian are placed two quadrants, and on the other side one, similarly divided as the meridian, and furnished with a similar index and nonius. These quadrants are movable on a pivot or hinge, rising perpendicularly from the centre of the horizontal plate, or agreeing to this centre; they are singly movable on the pivot, but capable of being attached at any relative distance, and retained in that situation by a screw, binding together tails attached for that purpose. To the east and west points of the horizontal plate is attached a horary circle, divided into hours, etc.
This horary circle represents the daily path of the sun, and it may be furnished with a nonius, as other parts are. This circle is so attached to the horizontal plate, that it can be moved parallel to it to suit the sun's declination; this is effected by the circle being attached to two tangent plates, which, by grooves, slide on the projections from the horizontal plate by means of screws passing through and working in these projections, and carrying the tangent plates, and with them the horary circle, to the degree of the sun's declination. This degree is indicated on a scale of tangent divisions on the tangent plates; and as such tangents are of various lengths, an expanding vernier is used to adjust them. Its expansion is effected by friction wheels, and springs working against a proper curve.