Poor Line of Position Computer Circular Slide Rule Retailed by Negus

This line of position computer, or mechanical navigator, is essentially a circular slide rule for determining one's location, either from morning or afternoon sightings for longitude or from the St. Hilaire method of finding the line of position. It has a circular black steel base, a green felt cushioning ring, an engraved brass disc, and a yellowed and warped celluloid disc. The metal and plastic arm that extends over the celluloid disc has a brass thumbscrew at the outer edge. The celluloid disc and arm can rotate together or independently.
The brass disc is calibrated logarithmically in several concentric rings: from 80 to 0 degrees by minute in both directions (altitudes, adjusted by latitude and declination), from 0 to 500 by 1 (numbers), from 100/10/1 to 600/60/6 (logarithms), from 0 to 12 hours (hour angles), from 0 to 70 degrees by 1 X 2 (declinations), from I to XII by I (hour angles), and from 0 to 180 degrees (altitudes and azimuths).
The computer is housed in a square wooden case (lock broken and handle missing) with doors that open from the top. It appears to be impossible to remove the computer from the case. Two copies of an instructional flyer are stored separately (MA*320413.1). These describe the computer as built in two sizes, for military aircraft and for battleships. It is not clear, though, whether the computer was able to place a ship close enough to its actual position (within 0.5 to 9.6 miles, according to the examples in the flyer) to be of use for military purposes around the time of World War I. The scarcity of surviving examples suggests the government and general public had little interest in the instrument. Indeed, aviators preferred inspection tables over slide rules for navigation.
The instrument is engraved near the center: LINE OF POSITION COMPUTER (/) DESIGNED BY (/) CHAS. LANE POOR (/) NEW YORK, U.S.A. (/) PATENT APPLIED FOR.
This computer was sold by the nautical instrument firm founded in 1850 by T. S. & J. D. Negus of New York City. It was invented between 1914 and 1918 (the date of his patent application) by Charles Lane Poor (1866–1951), who earned a Ph.D. under Simon Newcomb at The Johns Hopkins University in 1892. Poor taught at Johns Hopkins until 1899, when he took over his father’s print works in New Jersey. From 1903 to 1944, Poor was professor of celestial mechanics at Columbia University. He was a critic of Einstein’s theory of relativity and an avid yachtsman.
References: Charles Lane Poor, "Navigation Instrument" (U.S. Patent 1,308,748 issued July 1, 1919); Charles Lane Poor, Simplified Navigation for Ships and Aircraft (New York: The Century Co., 1918); Richard Berendzen and Richard Hart, "Poor, Charles Lane," Dictionary of Scientific Biography xi:83–84; National Cyclopaedia of American Biography xxxviii:614; Ronald van Riet, "Position Line Slide Rules: Bygrave and Höhenrechenschieber," https://sites.google.com/site/sliderulesite/position-line-slide-rules.
Currently not on view
Object Name
calculating rule
slide rule
date made
Negus, T. S. & J. D.
Physical Description
hardwood (case material)
brass (overall material)
celluloid (part material)
metal (part material)
overall: 42.5 cm x 42.5 cm x 5.7 cm x 42.5 cm; 16 23/32 in x 16 23/32 in x 2 1/4 in x 16 23/32 in
place made
United States: New York, New York
ID Number
catalog number
accession number
Science & Mathematics
Rule, Calculating
Navigation, Aeronautical
Slide Rules
See more items in
Medicine and Science: Mathematics
Slide Rules
Data Source
National Museum of American History, Kenneth E. Behring Center
Credit Line
Gift of Ferdinand Oberthol, Pupin Physics Laboratory
Additional Media

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