738 THE RAMSDEN DIVIDING ENGINE. 



was to measure the exact liuear distance between the observatory at 

 Greenwich and the observatory at Paris, which was satisfactorily 

 accomplished under the direction of General Koy. 



In January, 1788, Jesse Ramsden, who had twice before undertaken 

 the task of constructing an astronomical circle, began the one which 

 he completed August, 1789. 



His deatli occurred in 1800, at which time he was a member of the 

 Eoyal Society, Fellow of the Imperial Academy of St. Petersburg, and 

 wore a Copley medal. 



THE DIVIDING ENGINES OF TROUGHTON, f>IMS AND OTHERS. 



Eighteen years after the completion of Kamsden's engine (1793), Ed- 

 ward Troughton completed a circular dividing engine, somewhat similar 

 in detail, with a plate smaller than Rnmsden's. And in 1843, William 

 Sims, Troughton's successor, completed his engine, which has for nearly 

 50 years been in constant use at Charlton, near London. 



Sims claimed that the merit of this engine consisted in making the axis 

 of the plate a hollow tube into which the axis of the circle to be divided 

 could be slipped, not makiug it necessary to detach the plate while it 

 was being graduated, and obviating the necessity of resetting the cir- 

 cle on the axle, which is always liable to create error. 



Reichenbach, in Germany, and Gambey, in Paris, and Adie, in Edin- 

 burgh, also constructed dividing engines of merit. Reichenbach's was 

 for a long time unsurpassed in accuracy. Gambey's is now at Hotel 

 Cluny, Paris. 



The German method which admits of great accuracy under skillful 

 management, is performed by copying from a large circle, originally 

 divided with extreme precision ; over this circle the copy to be made is 

 fixed concentrically ; the degrees and minutes are cut into the copy by 

 the aid of the micrometer microscope fixed independently over the 

 divided circle. 



In 1818, Eepsold erected a circle at Gottengen and in 1819 Reichen- 

 bach erected one at Konigsberg. Pist-or and Martins of Berlin, con- 

 structed circles for Copenhagen, Albany, Leyden, Leipsic, Berlin, Wash- 

 ington Naval Observatory, and Dublin. Since the death of Martins, 

 Repsold constructed circles for Strasburg, Bonn, Williamstown, Mas- 

 sachusetts, and Madison, Wisconsin; Troughton and Sims doing the 

 work for Greenwich, Harvard, and Cambridge. 



The Altazimuth, 8 feet in diameter, now (1890) at Palermo Observa- 

 tory, was divided by hand by Ramsden 



In 1800, Troughton constructed the first modern circle for the observa- 

 tory at Blackheath. 



In the Philosophical Transactions, for 1809, in a paper by Trough- 

 ton on dividing instruments, p. 140, he states: 



" I now subjoin are-statement of the greatest errors of each of the 



