WATEUSroUTS. 



Fig. l. 



The various appearances exhibited by water-spouts are repre- 

 sented in fig. 1. 



Xo satisfactory theory has yet connected these phenomena with 

 the general laws of physics. 



60. If the surface of a sea, lake, or other large collection of 

 water were exposed to the atmo- 

 sphere consisting of pure air 



without any admixture of vapour, 

 evaporation would immediately 

 commence, and the vapour de- 

 veloped at the surface of the 

 water would ascend into and mix 

 with the atmosphere. The pres- 

 sure of the atmosphere would then 

 be the sum of the pressures of the 

 atmosphere, properly so called, 

 and of the vapour suspended in 

 it, since neither of these elastic 

 fluids can augment or diminish 

 the pressure of the other. 



The vapour developed from the surface of the water thus 

 mingling with the atmosphere, acquires a common temperature 

 with it. This vapour, therefore, receiving thus from the air with 

 which it is intermixed more or less heat, after having passed into 

 the vaporous state, is superheated vapour. It has, therefore, a 

 greater temperature than that which corresponds to its density, or, 

 what is the same, it has a less density than that which corresponds 

 to its temperature. Such vapour may therefore lose temperature 

 to a certain extent without being condensed. 



61. But if the same atmosphere continue to be suspended over 

 the surface of water, the process of evaporation being continued, 

 the quantity of vapour which rises into the air and mingles with 

 it will be continually increased until it acquires the greatest 

 density which is compatible with its temperature. Evaporation 

 must then cease, and the air is said to be saturated with vapour. 



If the temperature of the air in such case rise, evaporation will 

 recommence and will continue until the vapour shall acquire the 

 greatest density compatible with the increased temperature, and 

 will then cease, the air being, as before, saturated. 



But if the temperature fall, the greatest density of vapour 

 compatible with it being less than at the higher temperature, a 

 part of the vapour must be condensed, and this condensation must 

 continue until the vapour suspended in the air shall be reduced to 

 that state of density which is the greatest compatible with the 

 reduced temperature. 



