422 On Atmospheric Density and Pressure. 



has been most happily applied in the estimation of altitudes 

 (particularly those of mountains) by theTorricellian tube, or, 

 as it is more commonly called, the barometer*. The mer- 

 curial column decreasing as we ascend, the column of air 

 decreasing in" height, and of course decreasing in its ability 

 to support the former; at that height therefore at which the 

 mercury would no more rise but remain in its bed, the pres- 

 sure must either be at its limits, or too slight to raise so 

 dense a body as mercury : but if a rarer fluid were employed 

 at this height, the force of pressure might still perhaps sen- 

 sibly act on it until that period, when, from the want of 

 both density- and pressure in the air, its particles could no 

 longer keep together, and a vacuum must ensue : indeed 

 this I think (if possible) the only^true mode of ascertaining 

 the height of the atmosphere, because where there is atmo- 

 sphere there will be pressure also. 



However, the barometer does not appear calculated for 

 estimating great heights in the atmosphere, because there, as 

 was said before, its decrease of density does not often keep 

 pace with its decrease of pressure or gravity, as appears from 

 the observations of Bouguer and Don Ulloa, &c. on the 

 Andes, the upper regions being (from various causes) not 

 subject to the same laws as the lower ones. Hence, as in 

 the common indications of a barometer, the pressure, weight, 

 and density are more or less connected with each other,, 

 though in reality, and strictly speaking, distinct, as before 

 mentioned: consequently the equability of its ascension 

 must, at these high regions, be considerably diminished, 

 from these three powers being more or less at variance with 

 each other, and therefore its indications must \>t erroneous. 

 If the ratio of the mercury's descent for our ascent were 

 known, and was equable, the barometer might ascertain any 

 height, and we might easily know that at which it would 

 remain at its level. 



In the lower regions (the heights of mountains for in- 

 stance) the mercury, it is said, generally falls an inch 



* Barometer, however, in strictness, means a measurer of the weight of 

 the air. 



for 



