/ 



149 



merits, which it is stated, agree perfectly well with each other, 

 and in which there is an error of more than five hundred 

 toises, or a fourth part of the total height of the Peak. Dr 

 Heberden lived seven years at Orotava ; we must regret, 

 that he enters into no detail on the nature of the instruments 

 employed by him and Mr. Cross, or the values of the angles, 

 or the length and levelling of the base, on which the triangles 

 repose. The whole of these operations, which we have just 

 mentioned, deserve no more credit than those of Don Manuel 

 Hernandez * who asserts, that he found in 1742, by geo- 

 metrical measurement, the height of the volcano to be 2658 

 toises, and consequently 200 toises higher than Mount Blanc. 



We are indebted to Borda for the knowledge of the real 

 elevation of the volcano of TenerifFe. This excellent geo- 

 metrician obtained an exact result after having fallen into an 

 error, which he attributes to the negligence of one of his 

 coadjutors. He took three measurements of the Peak j two 

 geometrical, and one barometrical. The first geometrical 

 measurement f, executed in 1771, gave only 1742 toises ; 

 and as long as it was considered as accurate, Borda and 

 Pirtgre found, by operations made under sail, the height of 

 the Peak 1701 toises % . Happily, Borda visited the Canary 

 Islands a second time, in 1776, conjointly with Mr. de Chas- 

 tenet de Puysegur j and he then executed a more accurate 

 trigonometrical measurement, of whicji he published the re- 

 sult only in his Supplement to the Voyage of the Flora § . 

 We there find " that the principal cause of the error com- 

 mitted in 1771 had been a mistake in the indication of an 



* Borda, Voyage de la Flore, t. i, p. 88. 

 f Ibid, t. i, p. 89. 



% " All the parts of our work reciprocally confirmed each 

 other, and concurred in the same determination." Ibid. t. i, 

 p. 120. Journ. de Phys, 1776, p. 66 j and 1779, p. 129. 



§ Vol. i, p. 378. 



