THE RELATION OF METEOROLOGY TO HORTICULTURE. 13 



solid particle of dust it is clear that that explanation cannot be a correct 

 one ; and indeed it is not strictly correct to speak of the drops of water as 

 floating in the air, because in reality they are always gravitating slowly 

 towards the earth, and therefore the explanation we require is one which 

 will account for the slowness of their fall rather than one to account for 

 their not falling at all. 



The rate at which the water particles descend is not uniform for all 

 the particles alike, but is proportional to their size ; and in this fact we 

 find the explanation of the further fact that in their descent the particles 

 grow and form the relatively large rain-drops with which we are all 

 familiar. The weight of the particles is proportional to the cube of their 

 diameters, and therefore by doubling the diameter of a drop its weight 

 will become increased eightfold. The rate of descent of the smallest drops 

 is very small because their diameters and weight are very small, and if 

 the drops were all of uniform size they should all descend at the same 

 rate and without increase of size. But their size is not uniform, and 

 therefore the larger and heavier drops will fall more quickly than the 

 others, and overtaking those smaller ones which are in their path will 

 collide with them and combine to form larger drops. These enlarged 

 particles of water will then fall at a still faster rate, and by overtaking 

 yet other particles the process of augmentation will be again and again 

 repeated, and at an ever increasing rate. 



The ultimate size of the rain-drop is, however, governed by another 

 set of phenomena. The aggregation of water particles is held together by 

 their cohesive force, the surface of the drop forming a kind of skin, which 

 is capable of resisting a certain amount of strain before it will break. 

 But when a drop has attained the size and weight at which this limit is 

 reached it will break up into smaller drops, and this explains why, 

 although a rain cloud may be of enormous thickness and density, the 

 rain-drops which fall from it never exceed a certain size — probably not 

 more than one-twentieth, or perhaps in extreme cases one-sixteenth, of an 

 inch in diameter. 



In my previous lecture I explained that there is a definite maximum 

 amount of vapour which a given volume of air, at a given temperature, 

 can contain in the gaseous state. When that amount has been reached, 

 any further vapour which may be added will be deposited as water, 

 unless at the same time the temperature of the air be raised, when its 

 capacity for holding vapour will be at once increased. On the other 

 hand, if without changing the amount of vapour the temperature of the 

 air were to be lowered, the cooled air would no longer be able to retain all 

 the vapour present, and therefore some of it would become condensed into 

 water. 



Cooling of the air is, therefore, the primary cause of condensation and 

 of the formation of fog or cloud, and we may, therefore, consider briefly 

 how such cooling may be brought about^in nature. 



There are more ways than one in which it may be effected. Air may 

 be sufficiently cooled to produce condensation by coming into contact with 

 colder ground which has been chilled by radiation on a clear, quiet night, 

 and the ground mists and fogs, which commonly occur after a warm day, 

 particularly in autumn, are generally caused in that way. 



