{{Short description|Instruments used in surveys 1784 to 1853}} [[File:RamsdenRS theodolite of 1787.jpg|thumb|300px|Ramsden RS, the theodolite of 1787. Destroyed by wartime bombing in [[Southampton]] in 1941.]]
The '''Ramsden surveying instruments''' are those constructed by [[Jesse Ramsden]] and used in high precision [[geodetic survey]]s carried out in the period 1784 to 1853. This includes the five '''great''' [[theodolite]]s—great in name, great in size and great in accuracy—used in surveys of Britain and other parts of the world. Ramsden also provided the equipment used in the measurement of the many base lines of these surveys and also the [[zenith telescope]] used in latitude determinations.
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==The great theodolites== A total of eight such instruments were manufactured by Ramsden and others for use in Britain, India and Switzerland.<ref>{{harvnb|McConnell|2007}}Jesse Ramsden (1735–1800)</ref><ref name=insley>{{harvnb|Insley|2008}} The Great Theodolites</ref> Ramsden himself constructed three theodolites and a further two were completed to his design by Mathew Berge, his son-in-law and business successor, after Ramsden's death in 1805. Of the other instruments one was constructed by [[William Cary (instrument maker)|William Cary]] and the other two by the firm of [[Troughton & Simms]].
===The Royal Society theodolite=== In 1783 the [[Royal Society]] of London reacted to (unfounded) French criticism of [[Greenwich Observatory]] by seeking Royal assent to undertake a high precision geodetic survey, the [[Anglo-French Survey (1784–1790)]], between [[Royal Observatory, Greenwich|Greenwich]] and the established French survey stations on the other side of the [[English Channel]]. Approval having been granted, General [[William Roy]] agreed to undertake the work and he immediately approached Ramsden to commission new instruments. Three years later the "great" theodolite was delivered after a delay attributable to Ramsden's tardiness, workshop accidents and his predilection for continuous refinement—"this won't do, we must have at it again".<ref>[[#{{harvid|RamsdenMirror|1827}}|Ramsden the optician]]</ref> The instrument was paid for by the Crown and the King immediately presented it to the Royal Society; for this reason the theodolite is designated as the Royal Society theodolite, or Ramsden RS in short.
There is a complete description of this theodolite in the final report of the [[Anglo-French Survey (1784–1790)]].<ref>{{harvnb|Roy|1790}} pp135-160 The final report of the [[Anglo-French Survey (1784–1790)]].</ref> The instrument was large, {{convert|36|in|mm}} across and it was normally mounted on a stand which placed the sighting telescope between 5 and 6 ft. high. It weighed about {{convert|200|lb|kg}} and the accessories and cases weighed as much again. It travelled around Britain for over sixty years, in its own sprung carriage, to locations where it was hauled up mountains, church towers and even scaffolded steeples. [[File:RamsdenRS theodolite of 1787 microscope detail.png|right|350px|thumb|Detail of the micrometer microscopes.]] The horizontal circular scale was divided very accurately with divisions at 15 minute (of arc) intervals using one of Ramden's own [[dividing engine]]s;<ref>{{harvnb|Ramsden|1877}} The dividing engines.</ref> the marks on the {{convert|36|in|adj=on}} diameter scale would be about {{convert|1/6|in|mm}} apart. The position of the telescope could therefore be read to the nearest quarter of a degree by eye but the exact position between the divisions was read with the aid of micrometer microscopes fitted with adjustable cross wires in the focal plane, as shown. The threads of the screws were such that fifteen full turns moved from one scale mark to the next, ''i.e.'' 15 minutes, and since the scale on the adjusting knob allowed one sixtieth of a turn to be measured the resulting accuracy was within one arc second.
The instrument is also fitted with a vertical semi-circular scale to measure the elevations of distant stations and therefore a height difference. Cross wires similar to those used in the microscopes are fitted into the [[eyepiece]]; they are adjustable by a screw thread which allowed angles to be measured to within five arc seconds.
Typical distances in the Anglo-French Survey were less than {{convert|20|mi|km}}: at that distance one second of arc corresponds to lateral or vertical displacements at the target station of approximately {{convert|7|in|cm|abbr=on}}. No other theodolite could match this precision at that time. It was the first instrument to be able to measure the [[Spherical trigonometry#Area and spherical excess|spherical excess]] of large survey triangles.
After completion of the Anglo-French Survey this instrument was stored at the Royal Society, but in 1799 the [[Board of Ordnance]] requested its use for the [[Principal Triangulation of Great Britain]]. On completion of the Survey the theodolite was stored in the headquarters of the [[Ordnance Survey]] at Southampton where it was destroyed in the bombing raids of 1941.<ref>{{harvnb|Owen|Pilbeam|1992}}, page 9</ref>
===The Board of Ordnance theodolite=== [[Image:3foottheodoliteramsden.jpg|thumb|350px|Ramsden BO, the theodolite of 1791. Now in the [[Science Museum, London]]<ref name="ramsden-bo"> {{citation|title=Ramsden's three foot geodetic theodolite, 1792|url=https://collection.sciencemuseumgroup.org.uk/objects/co53377/theodolite-used-for-the-principal-triangulation-of-great-britain-geodetic-theodolite|publisher=Science Museum|access-date=2020-05-07}} </ref>]] In his report to the [[Royal Society]] in 1775 [[William Roy]] had noted the suitability of India as a location for both [[meridian arc]] and parallel arc measurements. To his delight the [[East India Company]] were willing to undertake such a venture and ordered a second great theodolite from Ramsden. It was ready in 1791 but Ramsden felt obliged to increase the price because of problems in its manufacture. To his surprise the company rejected his price and refused to purchase the instrument. It was bought by the [[Charles Lennox, 3rd Duke of Richmond|Duke of Richmond]] who, as Master of the [[Board of Ordnance]], had provided most of the finance for Roy's [[Anglo-French Survey (1784–1790)]]. The theodolite is designated as the Board of Ordnance theodolite, or Ramsden BO in short. Richmond's intention was to use the new theodolite on the extension of Roy's survey to the southern counties of Britain. The instrument was basically the same as the first with added refinements, mainly to the number and placement of the microscopes with their precision micrometer stages. It was in use until the completion of the [[Principal Triangulation of Great Britain]] in 1853 and it is now in the [[Science Museum, London|Science Museum]] in London.<ref name=ramsden-bo/>
There is a description of the improvements made to this theodolite in the account of the Trigonometrical Survey for the years 1791–1794 by Mudge, Williams and Dalby.<ref>{{harvnb| Mudge|Dalby|Williams|1795|pp=445–446}} Account of the Trigonometrical Survey for the years 1791–1794.</ref>
===Other Ramsden theodolites=== Ramsden made at least one other {{convert|3|ft|adj=on}} theodolite of which parts were discovered in Switzerland.<ref name=insley/> After his death his firm was inherited by Mathew Berge who is known to have constructed two more large instruments to Ramsden's design.<ref name=insley/> Ramsden made many theodolites including an eighteen-inch instrument of the same general design as the great theodolites.<ref>[http://www.scienceandsociety.co.uk/results.asp?image=10308271&itemw=4&itemf=0001&itemstep=1&itemx=7 Picture of Ramsden's 18 inch theodolite]</ref> This instrument and his two great theodolites are described in the account of the Principal Triangulation by {{harvtxt|Clarke|James|1858}}.<ref>{{harvtxt|Clarke|James|1858}} Principal Triangulation Section 2, page 43.</ref>
===Great theodolites by other builders === Although the East India Company turned down Ramsden's second theodolite, they commissioned a similar design from another London instrument maker, [[William Cary (instrument maker)|William Cary]]. This theodolite was in use in India from 1802 although there was a slight hiatus in 1808 when it was damaged when being hauled to the top of a building. It was repaired and in use for sixty years. A new theodolite to an improved design was made for the Indian Survey by [[Troughton & Simms|Troughton and Simms]] in 1830 and the two together saw service until 1874 when the Cary instrument was replaced by another by Troughton and Sims. This last instrument was a monster weighing {{convert|1455|lb|abbr=on}} when in its travelling cases: it was no surprise that it was deemed too heavy for transport up mountains and it passed to South Africa in 1882.<ref name=insley/> No more great instruments were made after 1874 for continuing advances in precision instruments eclipsed their performance: by the end of the nineteenth century an accuracy of 1 arc second could be obtained with a {{convert|12|in|adj=on}} instrument.<ref>{{harvnb|Turner|1983}} Chapter 14.</ref>
==Chains and rods== Eighteenth-century surveyors used [[Gunter's chain]]s which were 22 yards long (one [[Chain (unit)|chain]] with 100 links of 7.92 inches). Their accuracy was adequate for cadastral surveying but they were deemed insufficiently accurate for the [[Anglo-French Survey (1784–1790)]], Britain's first high-precision survey. Roy asked Ramsden to prepare a new chain of 100 links, each one foot in length.<ref>[http://www.scienceandsociety.co.uk/results.asp?image=10280167&wwwflag=2&imagepos=6 Picture of Ramsden's chain]</ref> He also asked Ramsden to prepare three precisely-calibrated wooden rods 20 feet long. These proved to be a failure because of fluctuations in length due to varying humidity; instead three calibrated glass tubes were used. The chain proved to be as accurate as the glass tubes, and it was in use for baseline surveys over the next thirty years.<ref>The chains were superseded by Colby's compensation bars. See {{harvtxt|Clarke|James|1858}} Principal Triangulation Section 5, page 200</ref> Later, even more precise measurements showed that the accuracy of Ramsden's {{convert|100|ft|m|adj=on}} chain was within about 3 inches in 5 miles. In actual use the chain was supported throughout its length by wooden trestles, and tensioned with a known constant weight. Its [[Thermal expansion|coefficient of thermal expansion]] was carefully measured so that temperature fluctuations could be taken into account. Full details (with plates) are given in Roy's account of the measurement of the [[Hounslow Heath]] baseline.<ref>{{harvtxt|Roy|1785}}. Measurement of the Hounslow baseline.</ref>
American surveyors sometimes also used a chain of 100 feet, also with 100 links, known as the ''engineer's chain''. The term ''chain'' in both cases usually refers to the measuring instrument rather than a unit of length, and distances measured are normally given in feet and decimal fractions of a foot (not inches).
Despite Ramsden's chain originating in the UK, Gunter's predominated there: "When a chain is spoken of without qualification, Gunter's chain is meant", noted [[William John Macquorn Rankine|Macquorn Rankine]]'s mid-[[Victorian era|Victorian]] ''A Manual of Civil Engineering''.<ref>{{cite book |first=William John |last=Macquorn Rankine |author-link=William John Macquorn Rankine |title=A Manual of Civil Engineering |date=1863 |publisher=Griffin Bohn & Company |location=London |pages=[https://archive.org/details/amanualcivileng05rankgoog/page/n38 18]–19 |edition=2nd |url= https://archive.org/details/amanualcivileng05rankgoog |via=[[Internet Archive]]}}</ref>
==The zenith telescope== [[File:Ramsden's zenith telescope.png|thumb|Ramsden's zenith telescope.]]
The [[zenith telescope]] constructed by Jesse Ramsden in 1802 was used to determine the latitude of many stations of the [[Principal Triangulation of Great Britain]]. This portable instrument was designed to bring observatory precision to fieldwork. The outer frame stood about {{convert|12|ft|abbr=on}} tall and the telescope mounted on an inner frame was {{convert|8|ft|abbr=on}} long. The telescope was restricted to observations within a few degrees of the zenith in order to prevent errors due to refraction. A complete description of the instrument is given by Pearson.<ref>{{harvtxt|Pearson|1829}} An introduction to Astronomy.</ref> <!--Traces of the theodolite support structure were still to be found many years afterwards at some remote survey points, such as at Soldiers' Lump, the summit of [[Black Hill (Peak District)|Black Hill]] in the [[Peak District]] of England.<ref>Wainwright, A. ''Pennine Way Companion'' ISBN 0-7112-2235-5 p.157</ref> -->
==See also== *[[Repeating circle]]
==References== {{reflist}}
==Bibliography== {{refbegin}} *{{cite book |year=1858 |first1=Alexander Ross |last1=Clarke |first2=Henry |last2=James |title=Account of the observations and calculations of the Principal Triangulation; and of the figure, dimensions and mean specific gravity of the Earth as derived therefrom. |publisher=G.E. Eyre and W. Spottiswoode |location=London |url=http://google.co.uk/books?id=H2gD6s3n8ZgC |oclc=757179661 |postscript = . [http://google.co.uk/books?id=pHTlAAAAMAAJ PLATES]. An excellent summary of the report was published in the [http://articles.adsabs.harvard.edu/full/1859MNRAS..19..194C Monthly Notices of the Royal Astronomical Society]. }} *{{cite conference |title=The Tale of the Great Theodolites |first=Jane |last=Insley |year =2008 |conference=[http://www.fig.net/ FIG] Working Week on ''Integrating the Generations''. |location= Sweden |url=http://www.fig.net/pub/fig2008/papers/hs03/hs03_01_insley_2838.pdf}} *{{cite journal |year = 1785 |last = Maskelyne |first =Nevil |title =Concerning the Latitude and Longitude of the Royal Observatory at Greenwich; With Remarks on a Memorial of the Late M. Cassini de Thury |journal = Philosophical Transactions of the Royal Society of London |volume =75 |pages =385–480 |url = https://archive.org/details/philtrans05243850 |doi=10.1098/rstl.1787.0018 |s2cid = 186209872 }} *{{cite book |year= 2007 |first=Anita |last=McConnell |title=Jesse Ramsden (1735–1800): London's leading scientific instrument maker |publisher=Ashgate Publishing Limited |location=Aldershot |isbn=978-0-7546-6136-8 |postscript = . There is a substantial [http://rsnr.royalsocietypublishing.org/content/roynotesrec/62/3/331.full.pdf review] by Richard Dunn. }} *{{cite journal |year = 1795 |last1 = Mudge |first1 =William |last2 = Dalby |first2 = Isaac |last3=Williams |first3 =Edward |title = An Account of the Trigonometrical Survey Carried on in the Years 1791, 1792, 1793, and 1794 |journal = Philosophical Transactions of the Royal Society |volume = 84 |pages = 414–622 |url =https://archive.org/details/philtrans00785875 |doi=10.1098/rstl.1795.0023 |doi-access = free}} *{{cite book |year=1992 |first1=Tim |last1=Owen |first2=Elaine |last2=Pilbeam |title=Ordnance Survey, map makers to Britain since 1791 |publisher=Ordnance Survey (HMSO) |location=Southampton |isbn=9780319002490 |oclc=28220563 |postscript = . Freely available online at the [http://www.ordnancesurvey.co.uk/docs/ebooks/map-makers-to-britain-since-1791/index.html Ordnance Survew, Owen and Pilbeam] }} *{{cite book |year= 1829 |first=William |last=Pearson |title=An Introduction to Practical Astronomy: Volume 2 |url=https://books.google.com/books?id=hVgoAAAAQBAJ&q=ramsden%27s+zenith&pg=PR23 |pages=534–554 |publisher=Cambridge University Press |isbn= 9781108064064 }} *{{cite journal |ref = {{harvid|RamsdenMirror|1827}} |year = 1827 |title =Ramsden the optician |journal = The Mirror of Literature, Amusement and Instruction |volume =10 |pages =80 |url = http://www.gutenberg.org/ebooks/9919 }} *{{cite book |year= 1877 |first=Jesse |last=Ramsden |title=Description of an engine for dividing mathematical instruments |publisher=The Commissioners of Longitude |location=London |url=https://archive.org/details/descriptionofeng00rams |postscript = . (The plates are incomplete.) }} *{{cite journal |year = 1785 |last = Roy |first =William |title =An Account of the Measurement of a Base on Hounslow-Heath |journal = Philosophical Transactions of the Royal Society of London |volume =75 |pages =385–480 |url =https://archive.org/details/philtrans01222507 |doi=10.1098/rstl.1785.0024 |doi-access =free }} *{{cite journal |year = 1790 |last = Roy |first =William |title =An Account of the Trigonometrical Operation, Whereby the Distance between the Meridians of the Royal Observatories of Greenwich and Paris Has Been Determined |journal = Philosophical Transactions of the Royal Society of London |volume =80 |pages =111–254 |url =https://archive.org/stream/philtrans00940584/00940584#page/n0/mode/2up |doi=10.1098/rstl.1790.0015 |s2cid = 186211548 }} *{{cite book |year= 1983 |first=Gerard L'Estrange |last=Turner |title=Nineteenth-century Scientific Instruments |publisher=University of California Press |url=https://archive.org/details/bub_gb_FaAYfJYVNXQC |isbn=0520051602 |postscript = . [https://books.google.com/books?id=FaAYfJYVNXQC&pg=PA246 Chapter 14], page 146, covers surveying instruments, }} {{refend}}
== External links == * [http://www.scienceandsociety.co.uk/results.asp?image=10280167&wwwflag=2&imagepos=6 Photograph of a section of Ramsden's chain], at ''Science & Society Picture Library''
{{DEFAULTSORT:Ramsden Theodolite}} [[Category:Surveying instruments]] [[Category:Obsolete scientific instruments]] [[Category:1780s introductions]] [[Category:1784 beginnings]] [[Category:1784 in science]]