VOL. XXXII.] PHILOSOPHICAL TRANSACTIONS. 549 



The Longitude of Buenos Aires, determined from an Observation made 

 there by Pere FeuilUe. By Edm. Halley, LL. D. F.R.S. N'' 370, p. 2. 

 rol. XXXII. 



I have, as occasion offered, collected such celestial observations as might be 

 of use to determine the longitudes of places on the sea coasts of the world; 

 in order to get as near as possible the outline, or true figure of the earth, 

 without which our geography of the inlands must necessarily be very insuffi- 

 cient. The Memoirs of the Royal Academy of Paris, afford a good number 

 of observations of this kind, and among the rest, there is one made at Buenos 

 Aires on the river of Plate, in South America, by Pere Feuillee, in his voyage 

 to Peru : who, in the Memoirs for the year 171 ', is said to have observed at 

 that place on the IQth of August, 17O8, the immersion of the star in the 

 southern foot of Virgo, marked a by Bayer, behind the obscure limb of the 

 moon. Being desirous to see what longitude might be deduced from this ob- 

 servation, I soon found that there was a fault in the day, and also in the star; 

 for K of Virgo was then nearly in 3 degrees of Scorpio, and the moon would 

 not be there till the next day, Monday the !20th of August; and the latitude of 

 A being about half a degree north, the moon at that longitude would be about 

 3 degrees more southerly than the star, and consequently far from eclipsing it; 

 for at that time the descending node was in the very beginning of Libra. Hence 

 I concluded it must be some other star, that Pere Feuillee observed eclipsed by 

 the moon. The day was certainly the 20th, and not the 19th of August, as 

 was evident by the place of the moon ; but as to the star, it was neither in the 

 Tychonic catalogue, nor yet in that more copious British catalogue of Mr. 

 Flamsteed; but turning over that of Hevelius, I found a star whose situation 

 agreed well with the observation, and was undoubtedly the star that was seen 

 to immerge behind the moon : the place Mr. Hevelius gives it, allowing the 

 precession of the equinox, was then \^ \^ 5&\, with 2° b\% south lat. It 

 remained then for me to be assured of the place of this star, and accordingly 

 on the 21st and 24th of December last, I got such observations by help of the 

 circumjacent stars, that I was assured the place of the star, which is a fair one, 

 of the 5th magnitude, was at that time, Tf)^ 1° 58' 40'', with 1° 54^4-, south lat. 

 being above 2" in long, and 3' in lat. more than Hevelius gives it. The hour 

 of this occultation is set down precisely 'J^ 5' 38'^ at Buenos Aires, the lat. of 

 the place being 34'* 35' south. Whence the altitude of the moon there was 

 then 42** 48', and the parallactic angle 76° 38', and the parallax in long. 40' 1 \" 

 to the west, and in lat. 9' 33'' to the north. So that the moon's observed place, 

 corrected by parallax, was x^ 2° 28' 4", with south lat. 2° 51%. To this place. 



