THE FOBCES THAT PRODUCE STORMS. 69 



of pressure. The degree, however, of that diminution in any 

 particular part, will depend not solely on the amount of vapour 

 condensed, and consequently of air heated, but also on the 

 height in the atmosphere at which these changes take place* 

 With a given amount of vapour condensed, the nearer to the 

 earth that the condensation occurs, the greater will be the 

 reduction of pressure on any particular point of its surface, 

 and the farther from the earth, the less the reduction of pres- 

 sure on any particular part. Hence it follows that in the 

 polar regions, where the cold of the surfjice is intense, conden- 

 sation of a small quantity of vapour produces a greater effect 

 on the mercury of the barometer than it does in the warm 

 tropical regions. In the latter regions the base of the column 

 of warmed air may be at a considerable height, and the total 

 reduced pressure of that base may be spread over a large area 

 of the surface of the earth, and may consequently affect each 

 particular part of that surface in but a small degree : whilst in 

 the former regions the base of the warm column may be close 

 to the earth, and the reduction of pressure may therefore be 

 limited to a small area, within which however the pressure 

 might be greatly reduced* 



Air is expanded by heat, say a 480th part of its bulk, for 

 every increase of 1° of temperature, and consequently it will 

 be expanded one-tenth part by an increase of 48° of tempe- 

 rature. It follows therefore that where an ascending mixed 

 mass of air and vapour reaches a mean temperature of 48°, 

 above that of the undisturbed part at the same elevation, the 

 ascending mass would be one-tenth lighter than the adjoining 

 part, a difference equal to the weight of three inches of mer- 

 cury, and sufficient to cause the heavier to press up the lighter 

 air with great force. On the whole it is contended, that if an 

 adequate portion of aqueous vapour be supplied to any ascend- 

 ing mass of the atmosphere, the laws of cooling of the gases 

 by expansion, and of heating them through condensation and 



