29 
Of course a raindrop shows none of the structural peculi- 
arities of the hailstone, and consequently we have not the 
same evidence of the manner in which raindrops are formed. 
But the explanation is sufficient, and there is apparently no 
other. 
Kaindrops cannot possibly have grown to the size with 
which they reach the earth by the condensation of the 
vapour of the air which they pass through, for the same 
simple reason as that just stated for hailstones, namely, that 
there is no way in which the heat developed by condensa- 
tion can be got rid of. The fact that the upper regions of the 
air from which the drops start are colder than those through 
which they descend, might, as has been supposed, cause the 
drop to grow by condensing vapour in the air through 
which it passes, but only to a very small extent, and one the 
limit of which may be easily estimated. 
Suppose the drop to start having a weight Wj_ and a tempera^ 
ture and on reaching the earth to have a temperature 
Then the increase in the quantity of heat in the drop would 
be (4 — nearly. This heat would be developed by the 
qjj 
condensation of a weight of water (^2 ~~^i) |qqq nearly, so that 
even supposing — i^i=:100° F., which it could not possibly 
be, the increase in the weight of the drop could not be one 
tenth. 
It is obvious also that the drop would not have parted 
with its heat to the air it passes through, for it is 
assumed to be colder than this air. Therefore the only way 
in which it could have parted with its heat would have 
been by radiation. Some heat might be lost in this way 
but only a very small amount, and one of which an approxi- 
mate estimate may be made. For after the drop had 
acquired a considerable size, say one hundredth of a foot in 
diameter, the time occupied in its descent would be very 
small. Assume this to be one minute, and during this time 
assume that the drop is 100 degrees hotter than the sur- 
rounding objects, although this is of course far beyond wliat 
