This is one of two identical 12-inch theodolites that the Instrument Division of the United States Coast and
Geodetic Survey made in the early 1890s, for use in the Survey's transcontinental triangulation. After a critical examination, the Survey concluded that, "With respect to design and perfection of mechanical construction," these theodolites "rank with the highest in their class." Moreover, "so far as purely instrumental and observational errors are concerned, a mean measured angle from four equidistant positions of the circle of either instrument will have a precision equal to the requirements of the highest order of primary triangulation."
This theodolite is composed of several different metals: cast iron for the base; hardened steel for the bearing center and sockets; brass for the frame; silver for the graduated horizontal circle; and aluminum where suitable, to reduce the weight supported on the centers. The beveled edge of the horizontal circle is graduated to 5 minutes, and read by micrometer to 5 seconds. The noted optician, John A. Brashear, made the telescope objective.
Ref: Edwin Smith, "Notes on Some Instruments Recently Made in the Instrument Division of the Coast and Geodetic Survey Office," United States Coast and Geodetic Survey Report for 1894, Appendix No. 8, pp. 265-275, and plate 1.