Gurley Cox's Stadia Computer Circular Slide Rule

This instrument consists of a plastic disc riveted to a plastic base. It is similar to a paper instrument of the same name (see 1987.0221.01).

The base, just outside the disc, has a logarithmic scale that represents readings in feet of a stadia rod used with a transit telescope. The base, just outside the disc, has a logarithmic scale that represents readings in feet of a stadia rod used with a transit telescope. The edge of the disc has two scales of functions of angles. Applying the first scale multiplies the stadia reading by 1/2 sin 2A, where A is the vertical angle of the transit telescope. This multiplication gives the difference in elevation of the transit and the stadia rod, in feet. The second scale multiplies the stadia reading by the square of cos A, to find the horizontal distance of the rod in feet.

The back of the instrument bears an advertisement for W. & L. E. Gurley, a maker of instruments in Troy, N.Y. A transit is depicted; it appears to be Gurley's Explorers [sic] precise transit. This was the smallest and lightest Gurley transit, shown in the Gurley catalogs for 1910 and 1912 (with a different image than is on this rule) and 1921 (with same image as on rule), but not the 1928 catalog. The 1921 catalog advertises the celluloid form of the Cox stadia computer and indicates that it sold for 75 cents.

A maker's mark at the bottom of the back of the computer is not legible, but the firm of Whitehead and Hoag of Newark, N.J., is known to have manufactured the instrument for Gurley in the second quarter of the 20th century.

References: W. & L. E. Gurley, Catalogue of Gurley Engineering Instruments (Troy, N.Y., 1921), 50; Florian Cajori, "A Stadia Slide-Rule," Engineering News 43 (April 5, 1900): 232; Laine Farley, "Whitehead & Hoag Celluloid Bookmarks," http://www.bibliobuffet.com/on-marking-books-columns-195/archive-index-on-marking-books/1039-whitehead-a-hoag-celluloid-bookmarks-053109.

Currently not on view