1858.] Meteorological Olservations on JParisnath Hill. 39 



upon it. The lower stratum will expand and the tensions or 

 elastic pressures of the air and the vapour in it will divide them- 

 selves, according to the proportion in which they are mixed, into 

 the pressure which they have to support, whether that pressure be 

 produced by the weight of dry air alone or of both air and vapour. 



Deducting, in the lower stratum, the elastic pressure of one of its 

 constituents from the whole pressure, the remaining quantity will 

 not represent the Mass or weight of the entire column of the other 

 constituent, but solely its local tension or elastic pressure. 



"When the equilibrium is locally disturbed by the watery vapour 

 being supplied in irregular quantities, or withdrawn by condensation 

 or the up-current, the tendency to restore it between the disturbed 

 body of air and the surrounding masses must, owing to the elasticity 

 of the medium, immediately come into play, and thus small inequali- 

 ties between the elastic pressures of neighbouring masses of air, 

 being distributed almost instantaneously over larger bodies, will not 

 be felt upon the barometer. 



In the locality where the disturbance takes place, consequently, 

 the diminution or increase of watery vapour will only alter the re- 

 spective proportions in which the elastic forces of both constituents 

 of the atmosphere share in supporting the whole pressure, without 

 sensibly altering the pressure itself. 



In consequence of the expansion of a volume of dry air, depen- 

 dent on an admixture of watery vapour with it, the density of the 

 mass in the space which was originally occupied will be slightly dimi- 

 nished, as a part of the air is displaced and replaced by an equal 

 volume of watery vapour, which is specifically lighter than air. This 

 diminution will create in the moist air, independently of the endea- 

 vour to put its elastic pressure into equilibrium with that of the 

 surrounding body already alluded to, a tendency to interchange its 

 position with that of specifically heavier masses. Such an exchange 

 cannot be accomplished in a moment, and is probably effected very 

 slowly, if we remember that the transition between very dry and 

 very moist air, laterally and in a vertical direction, must be gradual. 

 The process must be particularly retarded, where large masses are 

 concerned, and will not interfere with the more instantaneous equali- 

 sation of the elastic pressures. 



