168 



We are struck at seeing in this table,, that Mr. de Borda 

 found his barometer, at the summit of the Peak, four lines 

 lower than other observers ; and this, without any indica- 

 tions of the thermometer tending to explain why there was 

 so enormous a difference in tlie atmospherical pressure*. It 

 might be supposed, that the instruments were deranged dur- 

 ing the night, which the travellers passed at the Station of 

 the Rocks ; but we find it expressly noted in Messrs. de 

 Borda and Varela's journals, that, the day after the excur- 

 sion, the difference between Mr. Pasley's barometer at Oro- 

 tava, and those which had been made use of for the measure- 

 ment of the Piton, remained the same to nearly two tenths 

 of a line. The volcano of Teneriffe, like all other very 

 slender peaks, is undoubtedly but little adapted to disclose 

 the error of a barometric coefficient. Oblique winds sweep 

 along the rapid declivity of the mountain ; and it is to be 

 presumed, that, at the time when Mr. de Borda measured it, 

 a very violent ascending wind, or some other unknown de- 

 ranging cause, occasioned the barometer to fall. The wea- 

 ther had been rainy the preceding evening ; the decrement 

 of caloric was very slow and probably of very little uniform- 

 ity j circumstances under which any formula would be at 

 default ; but notwithstanding these considerations, without 

 the testimony of an observer so exact as Mr. de Borda, we 

 should scarcely believe that the barometric pressure could 

 change four lines at a height of more than 1900 toises, and at 

 the limits of the torrid zone. A single barometric measure- 



* The error of a degree in the indication of the temperature 

 of the air would alter the height of the Peak only 3*8 toises 

 nearly. A considerable number of good observations, made 

 at the top of St. Bernard, prove, that the whole of the cal- 

 culated elevations are too great or too little, every time that 

 the temperatures are above or below the mean temperature 

 of the two stations. Journ. de Phys, t. lxxi, p. 10. 



