Accession No
2253
Brief Description
slide rule for the determination of longitude by the lunar distance method, by Robert Brettell Bate, after the invention of David Thomson, English, 1820 (c)
Origin
London; England
Maker
Brettell Bate, Robert
Class
calculating; navigation
Earliest Date
1820
Latest Date
1820
Inscription Date
Material
wood (boxwood); metal (brass)
Dimensions
length 942mm; breadth 49mm; thickness 9mm
Special Collection
Provenance
Purchased from Turner and Devereux, London, 1976. Purchased with assistance a Grant-in-Aid administered through the Science Museum (PRISM).
Inscription
‘D. THOMSON INVt
BATE LONDON NO. 172’ (reverse)
Description Notes
Boxwood slide rule for the determination of longitude by the lunar distance method, by Robert Brettell Bate, after the invention of David Thomson, English, c. 1820.
Boxwood rule bound in brass with brass slider.
‘TIME SIDE’: identical scales for ‘Half Sum’ and ‘Latitudes’, former on stock, latter on slide. Divided 0 - 89˚ 50´, numbered 0, 10, 15, 20...80, 81, 82...89. 5˚ - 20˚ subdivided to 1˚, 20˚ - 30˚ subdivided to 30´, 30˚ - 50˚ subdivided to 15´, remainder subdivided to 10˚. Scale of ‘Difference’ on slide is an inverted version of these two scales. Lower part of stock carries two time scales for pm and am, former numbered [46 minutes] - 12 hours and the latter in reverse 12 hours - [23 hours 14 minutes]; each marked for every hour and for every ten minutes to 9 hours on upper scale, from 15 hours on lower scale. Upper scale: 46 minutes - 4 hours subdivided to 30 seconds; 4 hours - 6 hours subdivided to 1 minutes; 6 hours - 8 hours subdivided to 2 minutes; 8 hours - 11 hours subdivided to 4 minutes.
‘LUNAR SIDE’: upper part of stock has scale of ‘Appt Dist’ divided [28˚] - [89˚ 30´], 30, 35, 40...80, 81, 82...88; subdivided to 10´. Also scale of ‘Hor Par’ divided 53 - 62, numbered by 1, subdivided to 10´.
Slide: ‘Hor par’ repeated. Scale of ‘Appt Dist’ divided [28˚] -90˚, numbered by 5˚ except 85˚; 28˚ - 40˚ subdivided to 10´, 40˚ - 60˚ subdivided to 15´, 60˚ - 70˚ subdivided to 30´, 70˚ - 85˚ subdivided to 1˚. Also scale of ‘Appt Alt’ divided 5˚ - 90˚, numbered 5, 6, 7...20, 25, 30...80, 90; 5˚ - 20˚ subdivided to 5´, 20˚ - 40˚ subdivided to 10´, 40˚ - 60˚ subdivided to 15´, 60˚ - 70˚ subdivided to 30´, 70˚ - 85˚ subdivided to 1˚.
Lower part of stock: scale of ‘Correction’ divided [0] - 1˚ 40´, numbered 1´, 2´, 3´...20´, 30´...1˚ 40´; 0 - 20´ subdivided to 5´´, 20´- 50´ subdivided to 10´´, remainder subdivided to 15´´. Scale of ‘Com of Corr’ divided 20´ - 1˚ 59´, numbered 20´, 30´, 40´...1˚ 40´, 1˚ 41´, 1˚ 42´...1˚ 59´; 20´ - 1˚ 40´ subdivided to 5´, remainder subdivided to 30´´.
Condition good; complete
References
Events
Description
This slide-rule was made by the London scientific instrument maker Robert Brettell Bate (1782–1847). It is a navigational slide rule, used to calculate longitude at sea. For those without expensive portable timekeeping devices, calculating longitude remained difficult in the early 19th century. One of the most popular ways to determine longitude was the ‘lunar distance method’. After observing the moon’s position relative to a star with the aid of a sextant, a sailor could then use this rule to perform a series of calculations, enabling them to work out the difference between the local time and the time at Greenwich and hence determine the ship’s longitude. By sliding the components of the rule to reflect the moon’s perceived distance and altitude, and comparing the results to an almanac of data, a skilled operator could thus calculate his position east or west of the Greenwich Prime Meridian.
It is interesting to note that this method was still in use in the early 19th century, despite John Harrison’s development of an accurate portable chronometer by the 1760s. Such chronometers remained very expensive, and did not become widespread until decades later. The lunar distance method, which required only a sextant, slide rule, and almanac, was more affordable and accessible, despite the greater complexity of its calculations.
For more on Bate’s life and works, see Wh.6262.
20/12/2013
Created by: Joshua Nall on 20/12/2013
FM:42157
Images (Click to view full size):
