Bird's Bull's Eye Belting Computer Slide Rule by Whitehead & Hoag

This linear slide rule reflects changes that occurred in the materials of American manufacturing in the late 19th and early 20th centuries. The instrument itself has a pyroxylin (celluloid) envelope with a paper slide. There is no indicator. It was designed for use in computing the properties of belting used in industrial processes. At that time, such belting was typically made from one of three materials: canvas saturated with a liquid, leather, or rubber. The Boston chemical firm of J. A. and W. Bird and Company developed a new material that they called "Bird's Bull's Eye Belting," which consisted of canvas plies stitched together, with a gum base pressed around each cotton fiber. The maker claimed that this belting did not dry out (as the usual form of canvas belting did), resisted damage from fumes or humidity, and maintained its tension.

This instrument has two sides. The front, or "Computer for Belting," allows the user to find the revolutions per minute of a pulley, the speed of the belt in feet per minute, and the proper belt width for the horsepower, given the diameter of the matched pulley and its revolutions per minute. The back, or "Computer for Shafting" side, allows calculation of the horsepower a shaft can communicate, given the shaft's diameter and revolutions per minute. The calculation is made using Thurston's formula, which states that the horsepower equals the cube of the shaft diameter times the number of times it revolves per minute divided by a constant dependent on the nature of the shaft.

The instrument is marked on the front: COPYRIGHTED 1908 (/) BY J.A. & W. BIRD & CO. (/) ALL RIGHTS RESERVED BOSTON, MASS. On the back, it is marked near the left center in very small letters: PAT. JUNE 6, 1905. (/) THE WHITEHEAD & HOAG CO. NEWARK, N.J. Whitehead & Hoag manufactured a variety of plastic products including slide rules; see 1984.1080.01, 1987.0221.02, and 1988.0350.01 (which is also a belting computer).

References: Richard E. Roehm, "Process of Printing upon Pyroxylin Materials" (U.S. Patent 791,503 issued June 6, 1905); J. A. & W. Bird & Co., Belt Talks (Boston, 1909), 18. According to this publication, customers could receive one rule by sending 10 cents in stamps. A second rule cost 25 cents.

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