41 6 PHILOSOPHICAL TRANSACTIONS. [aNNO 1740. 



as M. Cassini himself acknowledges, who nevertheless approves and extols his 

 observations for their accuracy : so that those at the arctic circle may be very 

 good, notwithstanding the want of this, supposed necessary, operation. And 

 indeed, that they were so, sufficiently appears from this fact — The difference of 

 latitude between Tornea and Mount Kittis, found in September, was observed 

 again in March following, by the help of the same star S Draconis, and did not 

 differ from the former above 3^ seconds, though the instrument had been twice 

 carried from one place to the other. This is a degree of exactness not easy to 

 be met with ; no not in M. Cassini's observations, made on different stars, which 

 differ sometimes 40", in determining the amplitude of an arc in the heavens, 

 though the instrument was carefully examined in the way above men- 

 tioned. 



The author then proceeds, in his turn, to inquire into the accuracy and cer- 

 tainty of the two sets of observations made in the north and south parts of 

 France, in respect of the royal observatory at Paris. 



As to the measures of the degrees in the northern parts of France, between 

 Paris and Dunkirk, he owns that they cannot be much out of the way ; being 

 in some measure confirmed by M. de la Hire, in the year l683, and by M. 

 Cassini himself. Yet Mr. Celsius observes, that the basis on the sandy plain 

 shore, near Dunkirk, when measured again, differed three feet from the for- 

 mer measurement ; which is a much greater difference than that M. Celsius and 

 the other gentlemen found, in measuring a much longer line twice over, which 

 was but 4 inches. 



As to the astronomical observations taken by the 6-foot sector, whose limb 

 of 12 degrees was divided only at every 20" ; it is true, M. Cassini examined the 

 instrument several ways, at Paris, after his return thither ; but that a correc- 

 tion, owing to the change of centre, might be safely applied to the observations 

 at Dunkirk, the examen of the centre should also have been taken at Dunkirk ; 

 it being uncertain, whether this alteration or aberration of the centre, was caused 

 by the journey to or from Dunkirk. 



The difference of 41" between the observations taken to settle the true mea- 

 sure of the arc of the heavens, seems to be enormous. Perhaps the stars were 

 not lucid enough to be well observed by the 3-foot tube ; but might they not, 

 for a due degree of accuracy, have been viewed through the 9 or 10-foot te- 

 lescope ? 



Our author prefers the observations of 1719} made after the return to Paris, 

 to those made before ; because made at the same time of the year with those of 

 Dunkirk, and so not standing in need of Mr. Bradley's correction : though this 

 caution perhaps may be thought not necessary here, where the errors of the 



