VOL. LII.J 



the observation at the time, and 

 as it does not materially differ 

 from the others, he did not think 

 proper to reject it. 



^> 



PHILOSOPHICAL TRANSACTIONS. 



643 



iii,: 



After finding so great an agreement in the result of all the different observa- 

 tions, whether made on the same or different stars, or on the same or different 

 nights, he cannot account for the great difference found by the Abbe De la 

 Caille, in the result of several observations taken by himself, and a friend of his, 

 at land, which ought to agree still nearer with one another than those made at 

 sea. Mr. M. cannot conceive that such able observers could be liable to an error 

 of 5' in measuring the distance of a star from the moon's limb, if their instru- 

 ments were not faulty. The most likely and the most common cause of error 

 lies in the speculums and dark glasses; for if these are not ground truly parallel, 

 which he is afraid they very often are not, by the common methods, they may 

 easily produce a refraction of some minutes. 



As a proof how near different observations made in the compass of an hour or 

 two will agree in giving the same longitude, February 11 th, by 10 different ob- 

 servations of the distance of the moon from the sun, he made the longitude, 

 reduced to noon as usual, 28'' 57', 29° 50', 29'' 16, 29° 22', 29° 53', 28° 59' 

 29° 30', 29° 48', 29° 30', 29° 30'; none of which differ above half a degree from 

 29° 22', which is the medium of them all. March 18th, by 4 different obser- 

 j^rations of the distance of Pollux from the moon, he found the longitude 23° 52' 

 twice, and 24*^ 8' twice. He never found that a single observation would give 

 the longitude above a degree different from the medium resulting from 3 or 4 

 observations, and seldom above half a degree; which argues, that the error of 

 any single observation never exceeded 2 minutes, and seldom 1 minute. 



From the whole, Mr. M. congratulates the curious astronomer and ingenious 

 mariner, that the method of finding the longitude, proposed by Sir Isaac New- 

 ton, is by the improvement of the theory, of which he laid the foundation, and, 

 by the great perfection to which bur artists have carried the construction of in- 

 struments, rendered practicable in our times, at sea as well as at land, to a de- 



4n 2 



