VOL. LIII.] PHILOSOPHICAL TRANSACTIONS. 35 



made by M. Messier at Paris, he finds the difference of longitude between these 

 two places less, by 1 ™ of time, than between Paris and the observatory of M. de 

 la Caille at the Cape ; and therefore imagines that Mr. Mason's observatory was 

 to the west of M. de la Caille's. If M. Pingre had looked into the map of the 

 Cape by M. de la Caille, he would have seen that if Mr. Mason's observatory 

 had been 1*" of time to the west of M. de la Caille's, it must have been in the 

 ocean. Mr, S. is not at all surprised to see a difference or error of l™ of time 

 in deducing the difference of longitude between Paris and the Cape, by com- 

 paring Mr. Mason's observations with those of M. Messier : for he finds in the 

 last volume of the memoirs for 1761, a difference of J"" 5" between M. de la 

 Lande and M. Messier in an immersion of the first satellite of Jupiter, both of 

 these gentlemen observing at Paris, owing he supposes to the different goodness 

 of the telescopes used on this occasion ; for M. de la Lande says that he used an 

 18 foot refractor, the object-glass of which was tolerably good; and that M. 

 Messier made use of a very good reflector of 30 inches. If M. Pingre liad taken 

 the trouble of looking into the Philos. Trans., vol. 52, he would have found there 

 observations made at the Cape, and in Surrey-street, London, of the immersions 

 of the first and 2d satellites of Jupiter, with reflecting telescopes of equal good- 

 ness, of 2 feet focal length, where the difference of determination of the longitude 

 of these 2 places does not exceed one second in those of the first satellite, and 

 not above 16' in those of the second satellite. Mr. Mason's observatory at the 

 Cape was about half a mile to the south of M. de la Caille's, and about 10 or 12 

 yards to the west of the meridian of the same. 



M. Pingre also seems to think that the time shown by Mr. Mason's clock was 

 taken from a false meridian. When M. Pingre shall read the account given by 

 Mr. Mason of his observations at the Cape, which he says in his memoir he has 

 not seen, Mr. S. is persuaded he will be fully satisfied, from the many equal- 

 altitudes taken by Mr. Mason, that there can be no doubt of the times of his 

 observations being found from a true meridian. 



Mr. S. takes notice of a remarkable expression in the history of the memoirs 

 of the Royal Academy at Paris, p. 96, for the year 1757- It is there said, that 

 the English intended to send an astronomer to North America, to observe the 

 transit of Venus (according to the plan laid down by Dr. Haliey) before they saw 

 the map of the transit by M. de L'isle ; and the authority produced for this asser- 

 tion are the English news-papers, which, if they had understood the nature of these 

 papers, can be no authority at all. Mr. S. therefore on the best authority informs 

 the gentlemen who are the compilers of the history of these memoirs, that the 

 Royal Society never once thought of sending an observer to North America, even 

 before they saw the map of the transit by M. de L'isle. 



N. B. In this paper Mr, S. has employed the same elements as in his former 



p 2 



