22 



ON THE SEPABATE PKESSURES OF THE AQUEOUS AND 



height in the atmosphere, is heated and made very light. 

 Under these circumstances the pressure of the warmed column 

 on the surface must be materially reduced, and a barometer 

 placed under the column would, by its fall, shew the extent 

 of the reduction. But notwithstanding this reduction of total 

 pressure, the quantity of aqueous vapour in the column must, 

 under the circumstances described, be increased, the warmed 

 ascending current permitting more of it to remain uncon- 

 densed in the higher part than would have been possible at 

 that height v/ith the previous lower temperature : whilst the 

 cloud which is always formed at such times in the outer part 

 of the column would load the gases there with additional 

 globules of water, together making the aqueous part of the 

 atmosphere heavier. The heating and expanding of the 

 gases must, therefore, be sufficiently great to counteract the 

 increased weight of the aqueous matter, and, at the same 

 time, greatly to reduce its own weight. And to some such 

 heating as that just described we have to attribute the great 

 falls of the barometer, approaching three inches of mercury, 

 that sometimes occur; and when such a change takes place 

 in a whole column, it is obvious that the heavy adjoining air 

 will rush as a wind towards the comparative vacuum that 

 exists; but this rush, be it observed, does not occur until 

 after the vacuum has been formed : — the fall of the barometer 

 may therefore precede the rush of wind, or give warning of 

 the coming storm. 



This will also account for the fact, that the barometer 

 sometimes begins to rise during the storm. The heating and 

 expanding of the air takes place as soon as the first vapour is 

 condensed, and when only incipient cloud, not sufficiently 

 dense to be seen, is forming. And when the same process is 

 going on all around the observer, to a considerable distance, 

 as is common under such circumstances, it may be some time 

 before sufficient cloud is produced to become palpable to the 

 view ; for the cumulus cloud, when forming, like a gas jet in 



