344 



TRANSACTIONS OF THE 



very moist air, at the surface of the earth, continues to 

 flow towards the ascending cohimn. 



For moist air in ascending wiil constantly have some 

 of its vapour condensed and its latent caloric evolved ; 

 and thus its specific gravity diminished below that of the 

 surrounding air. While this process is going on, the 

 barometer will fall underneath the forming cloud, even 

 before it begins to rain ; for the air, as it expands in the 

 region of the cloud, will spread outwards, and thus di- 

 minish the quantity of gravitating matter over the region 

 below; and if the depression of the barometer is given, 

 the velocity of the upward motion of the air may be cal- 

 culated. 



I find, if the barometer stands one inch lower under 

 a forming cloud than it does in the surrounding regions, 

 the velocity of the air upwards will be 230 feet per 

 second. 



This velocity will be sufficient to carry up large drops 

 of rain after they are formed far above the region of 

 perpetual congelation and freeze them there, and then 

 carry them oif to the sides of the ascending column and 

 precipitate them in the form of hail. 



When the dew-point is very high and the ascending 

 column very narrow, the upward velocity will be very 

 great, and thus water spouts, or what the French call 

 trombes, both by sea and land, may be formed. 



In short, it is believed that all the phenomena of rains, 

 hails, snows and water spouts, change of winds and de- 

 pressions of the barometer follow as easy and natural 

 corollaries from the theory here advanced, that there is 

 an expansion of the air containing transparent vapour 

 when that vapour is condensed into water. 



It is now more than three years since I formed this 

 theory, and all the facts which I have been able to col- 

 lect since, particularly with regard to water spouts and 

 hail, have confirmed me in its correctness. 



