circular slide rule with Oughtred-type sundial reverse)
Date: 1633-1640
Inventory Number: DW0959
Classification: Slide Rule and Sundial
Subject:
Maker: English
Inventor: William Oughtred (English, 1574 - 1660)
Collector: Harrold Edgar Gillingham (1864 - 1954)
Collector: David P. Wheatland (1898 - 1993)
Cultural Region:
Place of Origin:
Dimensions:
instrument diam: 13.4 cm (5 1/4 in.)
Material:
Bibliography:
Of Time and Measurement
DescriptionThis instrument is in fact two instruments, and is one of the earliest known examples of Oughtred's invention, published in 1632. The obverse side is a circular logarithmic slide rule, also known as Oughtred's circles of proportions. It is made of eight concentric graduated circles, representing logarithmic scales for numerals, sine, and tangent and one for equal values.
Two separate rules are marked STTNETTS for the respective logarithmic and equal-value scales facing the marks. The two rules are used together to perform a multiplication or division using logarithms.
The reverse side is an horizontal instrument, a kind of sundial also invented by Oughtred. It consists of a stereographic projection of the celestial sphere onto the plane of the observer's horizon. The dial was made for a latitude of circa 53º 30', which would place the location near Leeds. There are several ways to determine the latitude. If you take the ruler and position it besides the PW (pole of the world) point, you get a measurement of circa 52.5 degrees. If, however, you turn the ruler 180 degrees and calculate the co-latitude at noon for the vernal equinox, then you get circa 36.5 degrees of co-latitude, or circa 53.5 degrees latitude north. The latter measurement should be considered best since it is based on the lines of declination.
At the center lies the zenith/nadir position from the observer point of view. Hours are marked on the instrument by vertical arcs radiating from the point PW, the pole of the world. The horizontal arcs crossing the hour lines are lines of declination. For the sake of clarity, the almucantars were not engraved on the instrument per se, but rather marked on the ruler that pivots at the center. PI and PII are the two poles of the ecliptic arcs. There is finally a degree scale engraved around the limb of the instrument.
Two separate rules are marked STTNETTS for the respective logarithmic and equal-value scales facing the marks. The two rules are used together to perform a multiplication or division using logarithms.
The reverse side is an horizontal instrument, a kind of sundial also invented by Oughtred. It consists of a stereographic projection of the celestial sphere onto the plane of the observer's horizon. The dial was made for a latitude of circa 53º 30', which would place the location near Leeds. There are several ways to determine the latitude. If you take the ruler and position it besides the PW (pole of the world) point, you get a measurement of circa 52.5 degrees. If, however, you turn the ruler 180 degrees and calculate the co-latitude at noon for the vernal equinox, then you get circa 36.5 degrees of co-latitude, or circa 53.5 degrees latitude north. The latter measurement should be considered best since it is based on the lines of declination.
At the center lies the zenith/nadir position from the observer point of view. Hours are marked on the instrument by vertical arcs radiating from the point PW, the pole of the world. The horizontal arcs crossing the hour lines are lines of declination. For the sake of clarity, the almucantars were not engraved on the instrument per se, but rather marked on the ruler that pivots at the center. PI and PII are the two poles of the ecliptic arcs. There is finally a degree scale engraved around the limb of the instrument.
In Collection(s)
Signedunsigned
FunctionOughtred's circles of proportions was invented to facilitate the use of logarithms. Instead of using Gunter's lines with a pair of dividers, this instrument, by simply moving the two rules, could make a multiplication or division more effectively. One still required printed tables of logarithms to convert numerals into logarithms, and vice versa.
The horizontal instrument was an easy-to-use sundial. By pointing it towards the sun and following a simple procedure, one could tell the time of day for a particular latitude.
The horizontal instrument was an easy-to-use sundial. By pointing it towards the sun and following a simple procedure, one could tell the time of day for a particular latitude.
Curatorial RemarksThough some of these instruments were made by Elias Allen, a then well-known London instrument maker and friend of Oughtred's, this one appears to have been made by a talented amateur. One, furthermore, that lived outside of London judging by the latitude for which this instrument was designed.
XRF analysis of the metals was done by Katherine Eremin and Sara Schechner in August 2007.
XRF analysis of the metals was done by Katherine Eremin and Sara Schechner in August 2007.
Primary SourcesWilliam Oughtred, The Circles of Proportions and the Horizontal Instrument, both Invented, and the Uses of both Written in Latine by Mr W. O., transl. by William Forster (London, 1632).
ProvenanceHarrold E. Gillingham Collection; David P. Wheatland, Topsfield, MA, 1949; gift to CHSI, c. 1985.