Patent Model for Estimator Slide Rule Invented by Fredric Maurice Stapff

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In 1874 F. M. Stapff submitted this model to the U.S. Patent Office. He designed the instrument to calculate the volume of spaces shaped as prisms, such as embankments and ditches. It consists of a wooden base with two slides, one atop the other. The lower slide has pieces of paper pasted on both sides, containing various curves related to finding the equivalent heights of an embankment (e.g., the height of an embankment of the same volume having trapezoidal sides).
The upper slide has five scales on it, numbered from 3 to 7. The upper part of the base has two scales, numbered 1 and 2. The lower part of the base has a scale numbered 8. Scales 1, 2, 3, and 6 are linear. Scales 4, 5, 7, and 8 are logarithmic. A single brass indicator is in a groove between scales 1 and 2; two smaller brass indicators are in a groove below scale 8. A wooden piece at the left end of the rule and a brass piece at the right end of the slide, with a sliding screw and sharp point, have screws for adjusting the rule. See the patent for a detailed account of the use of the instrument.
The lower part of the base is marked: F.M.STAPFF ESTIMATOR. A loose brass piece with the device may be set on "6 PT. Ct." or 7 PT. Ct." It is not mentioned in the patent. There is also a brown leather case that fastens with two straps and buckles. It is marked in Gothic lettering: F. M. Stapff's (/) Estimator.
Fredric Maurice Stapff (or Friedrich Moritz Stapff, 1836–1895) was a German-born geologist and mining engineer who studied at the University of Freiburg and obtained his Ph.D. from Jena in 1861. Stapff worked for a time in Sweden and in the United States, as well as in Germany and Switzerland. He also carried out extensive field work in East Africa, where he died while prospecting for gold. An example of Stapff's estimator was included among the slide rules exhibited at the Science Museum in South Kensington at the Special Loan Collection of Scientific Apparatus in 1876. According to the exhibit catalog, Stapff also patented his estimator in Sweden.
References: Fredric Maurice Stapff, "Improvement in Philosophical Instruments or Estimators" (U.S. Patent 157,239 issued November 24, 1874); "Improved Estimator," Scientific American n.s., 32, no. 2 (January 9, 1875): 25; South Kensington Museum, Handbook to the Special Loan Collection of Scientific Apparatus (Piccadilly: Chapman and Hall, 1876), 30; South Kensington Museum, Catalogue of the Special Loan Collection of Scientific Apparatus, 3rd ed. (London: George Eyre and William Spottiswoode, 1877), 2; Mary Gunn and L. E. W. Codd, Botanical Exploration of Southern Africa: an illustrated history of early botanical literature on the Cape flora : biographical accounts of the leading plant collectors and their activities in southern Africa from the days of the East India Company until modern times (Cape Town: CRC Press, 1981), 332.
Currently not on view
date made
Physical Description
wood (overall material)
brass (part material)
leather (case material)
overall: 1.5 cm x 16.2 cm x 7.1 cm; 19/32 in x 6 3/8 in x 2 25/32 in
ID Number
catalog number
accession number
Rule, Calculating
Patent Models
See more items in
Medicine and Science: Mathematics
Science & Mathematics
Slide Rules
Data Source
National Museum of American History


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