94 LECTURES 



ability. The computation of the French astronomers, based upon this 

 triangulation, placed the observatory of Paris 9' 21" to the east of 

 Greenwich. The computations of Colonel Mudge gave 9' 19". 4. In 

 the mean time, the difference obtained by Dr. Maskelyne from astron- 

 omical observation was 9' 20". And nearer than this, it was sug- 

 gested, the difference probably would never be known ; but the 

 electric telegraph was not then invented. With this new aid the dif- 

 ference is fixed at 9' 20". 63. This last is adopted by the English 

 Nautical Alman ic. 



The triangulation of Great Britain and Ireland now extends from 

 the iScilly islands, in latitude 49° 53', to the Shetland isles, latitude 

 60° 50" — a distance of about 750 miles. During the progress of the 

 work, commenced in 1791 and continued to the present time, seven 

 base lines have been measured. The one on Hounslow Heath has 

 been measured three times — once with deal rods, once with glass rods, 

 and once with a steel chain of exquisite workmanship, made by Rams- 

 den. The results were as follows: 



Length by deal rods 27,405.76 feet. 



Length by glass rods 27,403.38 " 



Length by steel chain 27,402.38 " 



The base on Salisbury plains, of between six and seven miles in 

 length, has been measured twice, first with Ramsden's steel chain 

 and subsequently with General Colby's compensation bars, the latter 

 measure exceeded the former by about one foot. Sir J. Herschel says 

 that the greatest possible error in the Irish line, near Londonderry, 

 of between seven and eight miles in length, is supposed not to exceed 

 two inches. 



11. Many other arcs of the meridian have been measured. Three 

 of them deserve to be mentioned, on account of their great extent and 

 the extreme accuracy of the work. 



The Jirst is the great French arc, commenced in 1791, under order 

 of the National Assembly. The object was to determine a new unit 

 of measure of length, the metre, which should be based on an invari- 

 able standard in nature. It was determined to make it the yo o^oVo o" 

 part of the meridian of the earth from the equator to the pole. This 

 magnificent enterprise was assigned to Delambre and Mechain. De- 

 lambre took the northern section, from Paris to Dunkirk. Mechain 

 took the southern, from Paris to Perpignan, and extended the same 

 to Barcelona, in Spain. This arc was subsequently extended, (1806,) 

 by Biot and Arago, to the island of Fomentera, in the Mediterranean. 

 The whole arc from Dunkirk to Fomentera covers about 12^° of lati- 

 tude, or about 850 miles. Delambre's base of verification, of about 

 7.3 miles, differed from its computed lengths by only about 11 inches. 



12. The second is the great Indian arc, commenced by Colonel 

 Lancton in 1802 at a place called Punnce, near Cape Comorin, the 

 extreme southern point of Hindostan, and extended northward 

 through about 10° of latitude (680 miles.) After the death of Lanc- 

 ton, in 1823, the work was continued by Captain Everest, who had 

 been his principal assistant, through an arc of nearly 11^° (797 miles.) 



