68 



ON THE OBIGIN AND NATURE OF 



is found in the atmosphere, and therefore great and extensive 

 atmospheric disturbances seldom take place in those parts ; 

 but in the tropical regions, where vapour is more abundant, 

 the phenomena that have been under consideration are often 

 exhibited in energetic action over a wide extent. It is 

 probably at an elevation that gives a temperature below the 

 freezing point, even in the warmed ascending currents, that 

 the fierce storms of the tropics generally take place ; and where 

 those storms are very violent in their character, the probability 

 is that condensation of vapour and freezing of water are suc- 

 cessively carried to a great height in the atmosphere, although 

 the commencement of the former of these processes may have 

 been in the middle, or even in the lower regions. With a 

 temperature and dew-point of SC^ at the surface, in an undis- 

 turbed atmosphere, we have seen that the freezing point is 

 reached at a height of four thousand eight hundred yards ; but 

 in a fierce tropical storm the vapour from the lower regions 

 may be carried very far above that height. The comparative 

 vacuum formed by a heated ascending current, vyhich had a 

 dew-point of 80*^ at the surface, and which is successively 

 supplied from below with equally saturated air, may produce 

 an ascent of not merely four thousand eight hundred yards, 

 but of ten thousand yards or more. And in portions of the 

 column the current must be rapid as well as continuous, taking 

 a large amount of vapour from the lower to the higher regions, 

 where its congelation as well as its condensation may finally 

 produce those great differences of temperature in adjoining 

 parts of the atmosphere that have been pointed out. 



It is obvious that an ascending column extending over a 

 considerable area, or in other words, having a large horizontal 

 diameter, being lighter than contiguous undisturbed air, will 

 press with proportionately less weight on the surface of the 

 earth on which it rests ; and a barometer placed in such a part, 

 would, by the falling of the mercury, measure the diminution 



