335 



seen through the telescope, resembles one of the brighter portions of 

 the milky way ; of the nebula major, which is brighter, more irre- 

 gular, and composed of a great number of different parts ; and of the 

 dark space on the east side of the cross, or the black cloud, as it is 

 called, which is occasioned by the almost total absence of stars. It 

 is remarked by the author, that neither of the two nebulae, major and 

 minor, are at present in the place assigned to them by La Caille. 

 He finds also that scarcely any nebulae exist in a high state of con- 

 densation, and very few even in a state of moderate condensation 

 towards the centre. Some have bright points in or near the centre, 

 many of wnich may be stars ; but the greater number of the nebulae 

 appear only as condensations of the general nebulous matter into 

 faint nebulae of various forms and magnitudes, generally not well de- 

 fined ; while many of the larger nebulous appearances are resolvable 

 into stars of small magnitudes. But whether nebulae are universally 

 thus resolvable, is a question of which our instruments are yet in- 

 competent to afford a direct solution, and in the discussion of which 

 we have only analogy as our guide. 



An Account of Trigonometrical Operations in the Years 1821, 1822, 

 and 1823, for determining the Difference of Longitude between the 

 Royal Observatories of Paris and Greenwich. By Captain Henry 

 Kater, V.P.R.S. Read January 31, and February 7, 1828. [Phil. 

 Trans. 1828, p. 153.] 



The first section of this paper contains a narrative of the pro- 

 ceedings of the Commission appointed for executing the object an- 

 nounced in the title. 



The first trigonometrical operations for connecting the meridians of 

 Paris and Greenwich were carried on by General Roy, in coopera- 

 tion with Messrs, de Cassini, Mechain, and Legendre, in the year 

 1790, an account of which was published in the Philosophical Trans- 

 actions of that year. In 1821, the Royal Academy of Sciences, and 

 Board of Longitude at Paris, communicated to the Royal Society of 

 London their desire that these operations should be repeated, and the 

 following Commissioners were nominated by these scientific bodies 

 for that purpose ; namely, Messrs. Arago and Mathieu, on the part 

 of the Academy of Sciences; and Lieutenant- Colonel Colby and Cap- 

 tain Kater, on the part of the Royal Society. 



The instrument employed in these operations was the great theo- 

 dolite of Ramsden, belonging to the Royal Society, and the same 

 which had formerly been used by General Roy. 



It was at first proposed to adopt as a base, some one of the distances 

 given by the trigonometrical survey of Great Britain, and to connect 

 it with General Roy's stations ; but it was found that the guns and 

 wooden pipes, which had marked these stations, had been either re- 

 moved or lost, so that the exact stations could not be immediately 

 ascertained. The signals used for connecting the stations upon the 

 coasts of England arid France were lamps with compound lenses, 



