THE GASEOUS PORTIONS OF THE ATMOSPHERE. 



21 



barometer^ in a short time, to fall as much as two or even 

 three inches ; and through the extreme mobility of the 

 materials that are in action, great difficulty has been ex- 

 perienced in explaining, or even in conceiving, what can be 

 the cause of such great and rapid falls. The difficulty, 

 indeed, is so great as to oblige us to have recourse to analogy, 

 and to apply the knowledge which we have obtained of the 

 nature of the smaller and the average changes, to the larger and 

 sudden ones ; — presuming, that when certain causes, operating 

 with a given degree of energy, produce a certain amount of 

 effect, the same causes, when operating with greater energy, will 

 produce a proportionately greater effect. Having seen that 

 lightening the gases, by the warming process of condensation, 

 of vapour existing within the mass of the atmosphere, was 

 the cause of the fall of the barometer when the changes were 

 small and regular, as well as when considerable averages were 

 taken, we may reasonably presume that the same cause is in 

 operation in great, irregular, and sudden changes. Let us, 

 then, endeavour, from a general view of the subject, to con- 

 ceive what may be the circumstances that lead to one of those 

 great falls of the barometer which sometimes take place. 



Say that, in a certain locality the atmosphere to a consi- 

 derable height is charged with as much vapour as it can hold 

 in an uncondensed form, and that in the outer portion of it, 

 or in adjoining parts, dissolving clouds exist which by their 

 evaporation cool the gases intermixed with them, and cause 

 the gases to descend. The uncooled saturated air being then 

 a little warmer and lighter than the cooled air, the former 

 will be pressed upwards by the latter, and some of the vapour 

 in the ascending column will be condensed. The whole of 

 the column will now be warmed and expanded by the con- 

 densation that takes place within itself, and the heavier 

 adjoining air will press and follow it up with greater velocity, 

 the process increasing in energy and rapidity as condensation 

 proceeds, until the whole ascending column, up to a great 



