174 



fore the expedition of the Boussole and the Espiegle, the 

 latitude of the Peak of TenerifFe was generally thought to be 

 28° 12' 54" (Maskelyne, Brit. Mariner's Guide, p. 17). 

 Cook found the Peak, by observations made under sail, 

 12' It" more to the South, and 29' 30" more to the west, 

 than the Mole of Santa Cruz. The geometrical operations 

 of Borda give with more exactness 11' 37 7 7 difference in lati- 

 tude, and 23 / 4 7/ difference in longitude. At the Mole, the 

 Peak has been determined by azimuths West 28° 55' South 5 

 the angle of apparent height being 4° 37'. Distance 22740 

 toises, supposing the elevation of the volcano to be 1904 

 toises. .Latitude of the Peak 28° 16 7 53" n Longitude 18° 

 59' 54''. I give here all that relates to this celebrated 

 mountain, in order to induce nagivators to verify results, 

 which are so important to nautical geography. 



Mr. de Borda is the only traveller, who has compared in 

 an accurate manner the dip of the needle at Santa Cruz, and 

 at the top of the Peak of TenerifFe. He found the latter 

 1° 15 ' greater (Manuscrit du Depot, Cah. 4). This in- 

 crease of the dip observed on the summit of a high mountain 

 is conformable to what 1 have several times remarked in the 

 chain of the Andes. It probably depends on some system 

 of local attractions ; but in order to form a right judgment of 

 this phenomenon, we should know with precision the dip of 

 the magnetic needle at the foot of the volcano, for instance 

 at the town of Orotava. The variation in 1776 was 15° 45' 

 at Gomera, 15° 50 ' at the Mole of Santa Cruz, and 19° 40* 

 toward the north west, at the brink of the crater. 



