26 



air, being saturated, is no longer able to absorb, and which becomes separated from it by 

 passing into the state of small vesicles." 



This is the way clouds form, and, as you will see by the following, it is but by a 

 continuation of the same process they are precipitated in rain. If the cold current which 

 has produced them from the warmer atmosphere continue to exert its condensing force, 

 or if a more saturated current arrive, the process goes on, and now becomes molecular ; 

 that is, the larger particles rapidly come together in still larger ones, the force of gravi- 

 tation begins to be felt, and the whole process is described by that great meteorologist, 

 Herschel, as follows : — 



" In whatever part of a cloud the original ascensional movement of the vapour ceases, 

 the elementary globules of which it consists being abandoned to the action of gravity, 

 begin to fall. By the theory of the resistance of fluids, the velocity of descent in air of a 

 given density is as the square root of the diameter of the globule. The larger globules, 

 therefore, fall fastest, and if (as must happen) they overtake the slower ones, they incor- 

 porate, and the diameter being thereby increased, the descent grows more rapid and the 

 encounters more frequent, till at length the globule emerges from the lower surface of the 

 cloud, at the vapour plane, as a drop of rain, the size of the drop depending on the thick- 

 ness of the cloud-stratum and its density." 



Now, if my readers have but followed these learned gentlemen through their 

 technicalities they have grasped this plain fact : — Rain is the precipitation from the air 

 of moisture which was more than it could, at the degree of heat to which contact with a 

 colder stratum of air had reduced it, hold in solution. And to show how elevations, 

 ■especially if wood-crowned, produce rain, any one can also easily see that if a saturated 

 current of air arrive at a mountain chain or other height, and have to rise into the colder 

 atmosphere above, getting colder one degree, according to the season, as they rise 200, 250 

 or 330 feet, as the air is the colder the higher we ascend, it must in consequence part with, 

 as rain, much of the moisture it carries. Let us- remember, too, that rain difiers from 

 cloud only in being formed of drops produced by the mutual- attraction of lesser drops, 

 which rapidly fall by force of gravitation to the earth instead of floating, as the smaller 

 particles of moisture composing the cloud had been, in the air. 



Dew. 



We^ will now travel onwards to another important point, and will do our best to 

 observe the operation of dew. To understand this we have simply to remember that the 

 earth's surface is heated in the day by the sun, and gets much colder at night, the heat it 

 has obtained being what is called radiated oflf. Some objects cool in this way much more 

 than others, some leaves of plants more than others. "Whatever it may be which cools 

 these, these cool the air next to them, and produce the same effect as the cold stratum of 

 air we have just been speaking of in connection with clouds ;' (when it strikes the warmer 

 stratum, that warmer stratum is cooled so much that it cannot hold all its moisture, and 

 cloud is formed, and afterwards, if the process be continued, rain) ; so the grass, the 

 leaves, paper, glass, wood, all these cool quickly by radiation, form the cool stratum here 

 and compel the air close at hand to part with its moisture to them. We see it in the 

 morning covering them in the form of drops and call it dew. 



