Chap. 46.] the Height of Mountains. 317 



commonly 5 5, if, however, either of the barometers 

 (lands at 30 inches, and the annexed thermometer 

 at 55, no reduction is to be made in the degrees 

 indicated by that barometer ; but if either of them 

 is at 30, and the thermometer below 55, we muft 

 add the expanfion the mercury in the barometer 

 would have experienced at the heat of 55. If the 

 heat mould, on the contrary, be above 55, we muft 

 fubftracl: the degree of expanfion which it gains by 

 that heat. Every degree of Fahrenheit's fcale produces 

 an expanfion of 00.304 of the barometrical inch, 

 when the barometer is at 30; when, therefore, the 

 thermometer is at 11 below or above 55% we muft 

 add in the former, or fubftract in the latter cale, eleven 

 times that number from the barometrical height. In 

 the fame manner it may be calculated, whatever is 

 the height of the barometer. When this matter is 

 afcertained, the height is eafily found by comparing 

 the two barometers and calculating the denfity of the 

 air in the higher regions according to the principles of 

 geometrical progreflion. 



The higheft mountains are thofe which are fituated 

 at or near the equator ; and the Andes are generally 

 allowed to be the higheft of thefe. Catopaxi, one of 

 the Andes, which was meafured by Ulloa and the 

 French academicians, was found to be rather more 

 than three miles above the level of the fea; whereas 

 the higheft point of the Alps is not above a mile and 

 a half. Mount Caucafus approaches neareft to the 

 height of the Andes of any of the Afiatic mountains. 

 The Pike of TenerirF, which has been fo much cele- 

 brated, is about a mile and a half in height. It is 

 an extraordinary circumftance, that the moon, which 

 is a body fo much fmaller than our earth, mould fo 

 greatly exceed it in the irregularities of its furface ; 



feme 



