564 



the following example. Let us suppose, that when the ap- 

 proximate height, corrected for temperature, amounts to 

 2700 feet (a height reached by several of our Irish moun- 

 tains), the mean value of tt, or the pressure to be used in the 

 iinal factor of the formula, is 27.3, and of the force of va- 

 pour, 0.3 of an inch, its value when the dew point is 43.6, 

 then the elongation of the aerial column resulting from mois- 

 ture is ^f = gLth of 2700 = 30 feet. It will, of course, 

 have been observed that the correction for aqueous vapour 

 differs from that for temperature in the circumstance of being 

 always positive ; and this coincides perfectly with the obser- 

 vation I have had frequent occasion of making, namely, that 

 in damp states of the atmosphere heights calculated by the 

 formulae in general use are all appreciably less than the truth. 



And here I may be permitted to observe, that the great 

 Laplace, in discussing the barometric formula, in his " Sys- 

 leme du Monde," has fallen into a slight oversight ; for as a 

 rude method of compensating for the effect of the aqueous 

 vapour present in the atmosphere, he proposes, that in ap- 

 plying the correction for temperature the coefficient of the 

 expansion of gases should be augmented from .00375, its 

 value for one degree Centigrade, to .004. Now this would 

 certainly produce the desired effect at.all temperatures above 

 32°; but as below 32° this equation is subtractive, the aug- 

 mentation of the coefficient, instead of diminishing, would 

 increase the error. The following is the passage referred to: 



" Les vapeurs aqueuses repandues dans I'atmosphere, 

 etant moins denses que fair, a la meme pression et a la 

 meme temperature, elles diminuent la densite de I'atmos- 

 phere ; et comme, tout etant egal d'ailleurs, elles sont plus 

 abondantes dans les grandes chaleurs ; on y aura egard en 

 partie, enaugmentant un peu le nombre .00375 qui exprime 

 la dilatation de Fair pour chaque degre du thermometre. 

 Je trouve que Ton satisfait assez bien a I'ensemble des ob- 

 servations, en le portant a 0,004; on pourra done fair usage 



