Plotting Scale Master, Signed John Shearson

This brass rule has three plotting scales, one with diagonal scales at both ends (graduated to 1/10" and 1/20") and two with diagonal scales at their right ends (graduated to 1/40" and 1/30"). The first plotting scale is 5" long and divided to 1/2", the second is 2-3/4" long and divided to 1/4", and the third is 3" long and divided to 1/3". A 12" scale of inches runs sequentially across the top of all three scales.

A decorative pattern is engraved along the top and bottom edges of the instrument. Flat projecting pieces at the left and right ends may have been used to hold rules to be divided on the pattern of this rule. The back of the rule is marked: John Shearson. Since Shearson is not listed in the Webster Signature Database, it is uncertain whether he was the maker or previous owner of this rule. The rule may have been made in England in the 18th century. The Smithsonian acquired it in 1961.

Reference: Adler Planetarium, Webster Signature Database, http://historydb.adlerplanetarium.org/signatures/; Gloria Clifton, Directory of British Scientific Instrument Makers 1550–1851 (London: National Maritime Museum, 1995).

Currently not on view