Protractor and Parallel Rule

This brass parallel rule has a semicircular protractor attached to the top blade. The protractor is divided to degrees and marked by tens from 10 to 90 to 10. A movable arm attached to the origin point of the protractor contains a vernier, which was intended to permit the measurement of angles to 5 minutes of accuracy. The hinges connecting the blades of the rule are straight. There is no maker’s mark.
Mathematician James McKenna gave this measuring instrument to the Smithsonian. He reported that an ancestor used it at Bedford, Pa., before 1800. A name, scratched on one of the tools in the set of drawing instruments (MA*310891) that accompanied this protractor, suggests that the ancestor was John A. Stuart, who surveyed a line in Bedford County on Wills Mountain that continues to bear his name.
Compare this instrument to 1978.2110.06.
Reference: Peggy A. Kidwell, "American Parallel Rules: Invention on the Fringes of Industry," Rittenhouse 10, no. 39 (1996): 90–96.
Currently not on view
Object Name
protractor and parallel rule
date made
late 18th century
Physical Description
brass (overall material)
overall: 1.1 cm x 14.7 cm x 9.5 cm; 7/16 in x 5 25/32 in x 3 3/4 in
place made
United States
ID Number
catalog number
accession number
Science & Mathematics
Parallel Rules
See more items in
Medicine and Science: Mathematics
Parallel Rules
Data Source
National Museum of American History, Kenneth E. Behring Center
Credit Line
Gift of James McKenna

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