436 YIEWS, &C. PLATEAU OF CA^AMARCA. 



as determined by Malaspiua and marked in the maps pub- 

 lished by the Deposito Hidrografico de Madrid, was 5 b 16' 53*. 

 The transit of Mercury over the Sun's disc, on the 9th of 

 November, 1802 (which I observed at Callao, the port of 

 Lima, from the Round Tower of the Fort of San Felipe), 

 gave for Callao. by the mean of the contact of both limbs, 

 5* 18' 16" 5; by the external contact only, 5* 18' 18* (79 

 34' 30"). This result, obtained from the transit of Mercury, 

 has been confirmed by Lartigue and Duperrey; and by ob- 

 servations made during Capt. Fitzroy's expeditions of the 

 "Adventurer" and the "Beagle." Lartieue fixed the longi- 

 tude of Callao at 5* IT 58"; Duperrey made it 5 h 18' 16"; 

 and Capt. Fitzroy 5* 18' 15*. After having calculated the 

 longitudinal difference between Callao and the Convent 

 of San Juan de Dios at lama, by carrying chronometers 

 from the one place to the other during four journeys, I found 

 that the observations of the transit of Mercury determined 

 the longitude of Lima to be 5 k 17' 51* (79 27' 45* W. from 

 Paris, or 77 6' 3* W. from Greenwich.) See my Recueil 

 d" 1 observations astron., vol. ii. p. 397, and Relation hist., t. iii. 

 p. 592. 



POTSDAM, June. 1849. 



