164 



lower than they really are* • and we may suppose, what is 

 confirmed by barometrical measurement, that the trigonome- 

 trical result obtained in 1776 is rather too small than too 



great. 



On resuming what has just been laid down from examin- 

 ing in succession the different elements, that enter into the 

 calculation of the absolute elevation of the Peak of TenerifFe, 

 it follows, that the trigonometrical measurement made by 

 Mr. de Borda is probably exact at least to a three hundred and 

 sixteenth of the total height; unless we suppose accidental 

 mistakes, owing to the negligence of the observers. 



I have no doubt but the same degree of exactness may be 

 obtained in very favorable circumstances from repeated mea- 

 surements made by the barometer 5 but it is difficult to judge, 

 amidst a few isolated observations, whether oblique winds, 

 or an unequal distribution of heat in the successive strata of 

 the air, have not altered the results. Of three barometric 

 measurements made by Messrs. de Borda, Lamanon and 

 Cordier, and calculated after the formula of Laplace, and the 



* Biot, on extraordinary refractions, in the Mem. de l'ln- 

 stitut, 1809, p. 157, 177, and 180. Mr. de Borda, like the 

 greater part of those geometricians who have measured the 

 depression of the horizon, has neglected to indicate the tem- 

 perature of the ocean i but we know that at this period the 

 air was at 25 degrees, and from the observations already 

 made, page 6G- — 80, we may admit, that the heat of the wa- 

 ter of the sea was from 20 to 21 degrees. Now, heights of 

 thirty toises, calculated on the supposition of a mean refrac- 

 tion of 008, and of a uniform decrement in arithmetical 

 progression, appear diminished three toises when there are 

 four degrees of difference between the temperature of the air 

 and the water. This number is deduced from the numerous 

 observations made by Messrs. Biot and Mathieu at Dunkirk; 



