356 HISTORY OF ASTRONOMY. 



the distance between this meridian and the present posi- 

 tion of the transit instrument of the Paris observatory. 

 "We thus have for the final result 9m. 20'51s. 



The difference of longitude between Greenwich and 

 Paris has been heretofore determined with all the 

 accuracy which science could supply; by eclipses of 

 the sun; by occultations of stars; by explosions of 

 rockets ; by geodetic triangulation ; and by the trans- 

 portation of chronometers. 



The first important measure was made in 1790, by a 

 triangulation conducted by General Eoy for England, 

 and by MM. Cassini, Mechain, and Legendre for France. 

 This measurement gave the difference of longitude 

 9m. 18-8s. 



The second geodetic measurement was made in 1821, 

 2 and 3, by Captains Kater and Colby for England, and 

 by French astronomers from Calais to Paris. The result 

 of this measurement was 9m. 21'18s. 



In 1825 the difference of longitude was determined by 

 fire signals. The operations were conducted by Messrs. 

 Herschel and Sabine for England, and MM. Bonne and 

 Largeteau for France. The result of this trial was 

 9m. 2146s. 



In 1838, Mr. Dent, of London, transported twelve of 

 his chronometers from Greenwich to Paris, and returned 

 them from Paris to Greenwich, having compared them 

 each time with the clocks at the two observatories. The 

 mean of the results furnished by these chronometers was 

 9m. 22*ls. in going, and 9m. 20'5s. in returning. 



