so 
could possibly be. According to the most accurate data 
the amount of heat it would then lose would not be suffi- 
cient to condense one hundredth of a grain of water^ an 
altogether unappreciable 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 rain drops 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 82° to be changed into a fog or cloud ; then if a 
particle, after commencing to descend, aggregated to itself 
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 size of drops. The diffi^ 
culty is rather the other way in explaining why the drops 
are not sometimes larger than they are. 
There are, however, two reasons why rain drops 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 
