166 



served at Orotava every quarter of an hour) its greatest 

 variations, in twenty-four hours, were a few tenths of a line. 

 The scales were carefully verified ; and an account was taken 

 of the accumulation of the mercury in the cistern*'. The 

 thermometer was observed in the shade j the slightest cir- 

 cumstances are found indicated in Messrs. de Borda and 

 Varela's journals. They are the only travellers, who have 

 carried two barometers to the top of the Peak. Both instru- 

 ments agreed within three or four tenths of a line with each 

 other, and the average of both was constantly taken. If we 

 were not acquainted with the real height of the Peak to a 

 considerable degree of exactness, we might presume, that 

 the barometric measurement taken in 1776 could not be a 

 hundredth erroneous, while it is probably beyond a fiftieth. It 

 is sufficient to compare the indications of Borda's barometer 

 and thermometer with the indications of these same instru- 

 ments in Lam anon and Cordier's voyages, to discover, that 

 in the morning of the 1st of October, 1776, on the summit 

 of the Piton, the pressure of the air underwent an extra- 

 ordinary and very problematic modification. The following 

 are the elements of this comparison. 



* It was 0 9 of a line on the brink of the crater. 



