t)42 PHILOSOPHICAL TRANSACTIONS. [aNNO I762. 



By the comparison of the longitude determined by the moon, with the longi- 

 tude by the common reckoning, they seemed to have been set by a cuiTent to 

 the eastward about 20 miles per day, between February 10th and 15 th, while 

 they were passing from 17^* to 5" north latitude, at the distance of about 11° 

 westward of the coast of Guinea; and he was told that ships passing near the 

 coast of Guinea always meet currents, which set them in on the land, which 

 are so much the stronger the nearer they approach the coast ; as on the contrary 

 if they approach the opposite coast of the Brasils, they will be set by a current 

 to the westward, which appeared to have been their case ; for between February 

 1 Qth and 28th, during which time they passed by the most eastern part of the 

 coast of Brasils, leaving Cape St. Augustin only 6'^ to the west, they appeared 

 to have been set by a current, at the rate of 20 miles per day to the westward, 

 and from this time to their arrival at St. Helena, they seemed to have been con- 

 tinually set to the westward, though slower than before; which must have been 

 owing to their approaching so much nearer, and continuing so much longer near 

 the eastern coast of South America, than the western coast of Africa. 



Though Mr. M. had no observation of the moon within less than 8 days of 

 his arrival at St. Helena, he makes the longitude of the island, by his account, 

 to be only 1-^° east of its true situation, which is 5|-° west of London; whereas 

 the account kept in the common manner made the island 1^° east of London, or 

 74-° east of its true situation, and most of the accounts on board the ship made 

 it 10° east of the true longitude. Having got 12 observations in the compass 

 of 1 1 days, between March Qth and 20th, he had the curiosity to compare them 

 together. Setting aside any errors in the common method of keeping a ship's 

 reckoning, and supposing her not to be affected by currents during this time, 

 the same difference ought to have been found between all the longitudes by ac- 

 count, and the longitudes deduced from the moon, or the same error of the 

 common account ought to have resulted from all the observations. The mean 

 error of account from all the 12 observations is 5° 20', by which they were really 

 more to the west than the account made them. And comparing each particular 

 error of account with this quantity, the difference between them, in any of the 

 12 observations, scarcely exceeds a degree; whence we may suppose, that the lon- 

 gitude was deduced truly from every one of these 12 observations, within the 

 compass of less than 1 -J-°. 



Mr. M. has set down in the annexed table the error of the common account, 

 and the difference between 5° 20', the mean quantity of it, and each particular 

 error of account, which, except in the first and last observations, does not ex- 

 ceed -%°. The last observation, which differs most from the medium, was taken 

 in some haste, on account of the position of the sails of the ship, which did not 

 allow an uninterrupted view of the star; yet as he was tolerably satisfied with 



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