Patent Model for Triangular Drafting Scale Invented by Josiah Lyman

Both edges of one side of this German silver triangular rule have scales divided to 1/40" and numbered by twos from 3 [ sic ] to 24 and by ones from 1 to 6. This side is marked: In. 40. U.S. STD. It is also marked: Josiah Lyman, Lenox, Mass.

Both edges of the second side have scales divided to 1/24" and numbered by ones from 1 to 12 and from 1 to 6. This side is marked: In. 24. U.S. STD. Both edges of the third side have scales divided to 1/20" and numbered by ones from 1 to 10 and from 1 to 5. This side is marked: Ft. 200. U.S. STD.

One end has a micrometer screw. According to the patent, there should be three interchangeable micrometer circles, but only one is present. The other end is supposed to have a thumb-nut, but the tube that would hold it is broken. Three large holes are in the center of each side. Three pinholes are in each end.

Among other inventions, Josiah Lyman (1811–1889) patented a protracting trigonometer (U.S. Patent 20,356 issued May 25, 1858). See MA.328738. This triangular rule was supposed to have a brass spring attachment that would hook into the pin holes at each end of the rule and then around the trigonometer, making the rule a sliding scale for the trigonometer. Lyman argued that placing a weight on the arm of wire stretching across the trigonometer would make the sliding scale mathematically reliable. He claimed that the micrometer scale, together with the protracting trigonometer, could measure off the twelve-thousandth part of a line 12 inches long.

References: Josiah Lyman, "Improvement in Drafting Scales" (U.S. Patent 38,904 issued June 16, 1863); Peggy A. Kidwell, "Josiah Lyman's Protracting Trigonometer," Rittenhouse 3 (1988): 11–14.

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