404.—Solar Attachment to a Theodolite.—This appliance is an adaptation to the theodolite of the solar compass of W. A. Burt, of Michigan, which was made to replace the magnetic compass in determining a true meridian, or north and south line, by observation of the sun only. It was brought into general use in the surveys of the United States public lands. The solar compass consists mainly of three arcs of circles by which the latitude of a place, the declination of the sun, and the hour of the day can be set off. In the solar attachment to the theodolite the latitude arc is found unnecessary, as this is formed by the vertical arc of the theodolite; therefore the hour and declination arcs need only be described.
Fig. 166.—Burt's solar attachment to a theodolite.
Larger image
405.—The Hour Circle, Fig. 166, H is fixed upon the centre of the telescope upon a socket axis S, which is placed perpendicularly to the optical axis and to the transverse axes or pivots of the theodolite. This circle is divided to read five minutes of time, and is figured I to XII twice, or I to XXIV, the index being a fine line carried down on a plate from the lower arm of the declination arc, which is fixed to the socket S. The hour circle, when set to any reading, may be clamped to this position by means of the milled head placed over the socket M.
406.—The Declination Arc is of 5 inches radius, divided to read on the same plane with a vernier V to single minutes of arc. The vernier arm is fixed by a clamp at C, which carries tangent adjustment T. At the back of the vernier arm two spur-pieces are carried out directly from it, L and I. These are blocks of metal about 1½ by 1¼ by ¼ inches, which carry each a lens of a focus L to I, and a silver plate to be presently described, upon which the sun's image is received in one direction or the other.
Fig. 167.—Image plate of solar attachment.
Larger image
407.—The Image Plate, Fig. 167, is marked with two sets of lines intersecting each other at right angles. The lines bb are termed hour lines, the lines cc equatorial lines; these lines having reference respectively to the hour of the day and the position of the sun in relation to the equator. The intervals between the lines bb and cc are just sufficient to include the circular image of the sun formed by the solar lens on the opposite end of the vernier arm. The axes of the solar lenses and corresponding image plates are placed parallel with each other, and with the direction of the vernier arm. Below the lower line c three other lines are cut at 5 minutes apart. These are useful for making allowance for refraction. The following description for the use of the instrument is partly extracted from Messrs. Gurley's manual.