WHICH RAINDROPS AND HAILSTONES ARE FORMED. 59 



downward path are a necessary and sufficient cause ; but 

 the origin of the drops and stones requires further expla- 

 nation. Why should some of the particles in a cloud be 

 larger than the others^ as it is necessary for them to be in 

 order that they may commence a more rapid descent ? 



A cloud does not always rain ; and hence it would seem 

 that in their normal condition the particles of a cloud are 

 all of the same size and have no internal motion, and that 

 the variation of size is due to some irregularity or disturb- 

 ance in the cloud. 



Such irregularity would result when a cloud is cooling 

 by radiation from its upper surface. The particles on the 

 top of the cloud being more exposed would radiate faster 

 than those below them ; and hence they would condense 

 more vapour and grow more rapidly in size. They would 

 therefore descend and leave other particles to form the top 

 of the cloud. In this way we should have in embryo a 

 continuous succession of drops. 



Eddies in the cloud also form another possible cause of 

 the origin of drops and stones. Whenever the direction of 

 motion of a portion of the cloud is not straight, the sus- 

 pended particles will have more or less motion through the 

 air. And if, as in an eddy, the motion of the cloud varies 

 from point to point both in direction and magnitude, then 

 the motion of the particles through the air will also vary, 

 and they may overtake one another and, combining, form 

 larger particles or drops in embryo. 



Whatever may be the cause of the variation in the size 

 of the particles which form the cloud, we may know from 

 observations on fogs that such variations do exist, [n fogs 

 we have particles of all sizes, from those which are too fine 

 to be seen even by the aid of a microscope, and which will 

 remain suspended for hours without any appreciable de- 

 scent, up to such a size that they can be easily detected 



