334 Rain. 



other hand, a difficulty depending on the state into which water- 

 particles at high elevations sometimes pass. iC It is certain," 

 he says, " that they possess, on or after precipitation, the 

 power of building themselves into crystalline forms ; they thus 

 bring forces into play which we have hitherto been accustomed 

 to regard as molecular, and which could not be ascribed to the 

 aggregates necessary to form vesicles." 



In whatever state the particles of a cloud really exist, it is 

 certain that the fall of rain depends on a process of increased 

 condensation. The causes producing such condensation have 

 been thus summed up by Professor Nichol :■ — 



(1.) The cooling of clouds through the effect of radiation 

 from them; 



(2.) The mingling of vapours at different temperatures — a 

 mingling effected by the agency of the winds ; 



(3.) The rising of vapours towards colder strata of the 

 atmosphere ; 



(4.) The increase of atmospheric density or pressure ; 



And (5.) The accumulation and impinging of masses of 

 vapour against some obstacle. 



Singularly enough he omits the most important of all known 

 agencies in the production of rain, viz. : — 



(6.) The transfer from the equator towards the poles of 

 large masses of moisture-laden air by means of the upper S.W., 

 or counter trade-winds. 



I must note also that cause (4) is more than doubtful. 

 Tyndall has shown that rarefaction is an efficient agent in pro- 

 ducing the precipitation of vapour. By increase of pressure a 

 larger quantity of moisture is, indeed, compressed within any 

 given space ; but, on the other hand, there is an increase of 

 heat within the space which more than counterbalances the 

 former in effect. "The heat developed," says Tyndall, speak- 

 ing of an experiment illustrating the effects of increased pres- 

 sure, " is more than sufficient to preserve it" (the moisture 

 added to a given space) " in the state of vapour." 



It will be seen at once from the above imperfect enumera- 

 tion of causes affecting the production of rain, that the phe- 

 nomenon is no simple one. When we add the variety of cir- 

 cumstances affecting the action of different causes — as the 

 latitude of the place, the elevation above the sea-level ; the 

 proximity of the sea ; the laws affecting the seasonal variations 

 at the place ; the prevailing wnids ; and the configuration of 

 the surrounding surface, it will become evident that meteo- 

 rologists may well be perplexed by the very complex set of 

 agencies acting in the production of rain ; and so fail — as they 

 have hitherto dono — in interpreting any save the most general 

 laws influencing the phenomenon. 



