WHICH RAINDROPS AND HAILSTONES ARE FORMED. 57 



inappreciable amount when compared with the weight of 

 the drop, which would be nearly the quarter of a grain. 



It appears clear, therefore, that the only way in which a 

 falling drop can grow is by the aggregation to itself of the 

 particles of moisture in the air; and the only way in which 

 it can encounter these is by its downward motion through 

 this air. 



Such a means of growth is amply sufficient to account 

 for the size of raindrops or of hailstones. 



If we suppose all the vapour which a body of saturated 

 air at 60° F. would contain, over and above what it would 

 contain at 30°, to be changed into a fog or cloud, then, if 

 a particle, after commencing to descend, aggregated to it- 

 self all the water suspended in the volume of air through 

 which it swept, the diameter of the drop after passing 

 through 2000 feet"^ would be more than an eighth of an 

 inch, and after passing through 4000 feet a quarter of an 

 inch, and so on ; so that in passing through 8000 feet of 

 such cloud it would acquire a diameter of half an inch. 

 Now, as clouds must often contain more water than what 

 is here supposed, there is no difficulty in explaining the 



This is equivalent to the latent heat of condensation of '0036 grain of water. 

 Again, let w be the weight of the drop ; then 

 w = 7000 X 62-5 X^T?"' 

 = '2 1 or nearly 5^ grain. 

 * If X be the diameter of the drop after descending a distance k, and p 

 the Tolume of water suspended in a unit volume of air, then the increase 

 of volume of the drop in descending a distance dk is given by 



— x^dx^p — x'^dh ; 

 * 4 



.■.dx=—dh; 



2 



or x=—h. 



2. 



Hence, if p=:ooooi, ar='ooooo5A ; 



and if .t = 'oi, A = 2000 feet. 



