ATMOSPHEROLOGY. — 'THEORY OF RAIN. 421 



sion the precipitation of any rain. But, from the 

 beautiful theory of the late Dr James Hutton, 

 supported by the able and ingenious researches of 

 Professor Leslie, it appears, that " while the tem- 

 perature advances uniformly in arithmetical progres- 

 sion, the dissolving power which this communicates 

 to the air, mounts with the accelerating rapidity of 

 a geometrical series ^ ;" and this, in such a ratio, 

 that the " air has its dryness doubled at each rise 

 of temperature answering to fifteen centesimal de- 

 grees," or twenty-seven of Fahrenheit f. Hence, 

 " whatever be the actual condition of a mass of 

 air, there must always exist some temperature at 

 which it would become perfectly damp J;" and hence, 

 whenever two streams of air, saturated with mois- 

 ture, of different temperatures, are mixed together, 

 or brush against one another, in the form of diffe- 

 rent currents of wind, there must always be a quan- 

 tity of moisture precipitated. For, if two masses of 

 air of different temperatures, but equal in quantity, 

 and both saturated with moisture, were mixed to- 

 gether, the resulting temperature would be nearly 

 the mean of the two ; but, at that temperature, the 

 capacity of air for moisture, being less than the 

 quantity contained in the two commixed masses, 



. " Leslie ; — " A Short Account of Experiments and Instru- 

 ments, depending on the relations of Air to Heat and Mois-!- 

 •ture," p. 123. 



f Idem, p. 122. . t Idem, p. 123. 



