58 PROF, OSBORNE REYNOLDS ON THE MANNER IN 



size of the drops. The difficulty is rather the other way, 

 in explaining why the drops are not sometimes larger than 

 they are. 



There are, however, two reasons why raindrops do not 

 acquire the full size which might be expected on the above 

 assumptions. 



In the first place, the drop will not aggregate to itself all 

 the particles in front of it. Some of these will be swept 

 away sideways by the diverging current of air ; and the 

 smaller the particles are the more will this be the case. 

 This is, of course, true for hail as well as for rain. 



The second reason applies only to rain, and explains why 

 it is that hailstones sometimes acquire magnitudes never 

 approached by raindrops. 



A drop retains its form simply by the surface-tension of 

 the water ; and as this is the same whatever may be the 

 size of the drop, its power to hold the drop together dimi- 

 nishes as the size of the drop increases, whereas the velo- 

 city and consequent tendency of the air to disturb the 

 shape of the drop increase with its size. Hence it must 

 eventually arrive at such a size that it can no longer hold 

 together, but will be blown to pieces by the rush of air 

 past it. This action may be seen in a waterfall or a fountain, 

 where, in passing through the air, a solid column of water 

 is separated into drops not larger than large raindrops. 



The same reasoning does not hold for hailstones, which 

 are held together by the adhesion of the particles through- 

 out their entire mass, and whose compactness and strength 

 increase with their size. It is, however, the case that the 

 smaller end of the stone, where the texture is looser, ap- 

 pears to be blown off in its subsequent descent, especially 

 when the stones acquire a larger size. 



It seems, therefore, that, so far as the growth of a drop 

 or a stone is concerned, the particles it overtakes in its 



