WITH ELEVATION. 
307 
commencement to the end of its descent, as it traverses suc- 
cessively the humid strata of air at a temperature so much 
lower than that of the surrounding medium as to cause the 
deposition of moisture upon its surface. This hypothesis 
takes account of the length of descent, because in passing 
through more air more moisture would be gathered ; it agrees 
with the fact that the augmentation for given lengths of descent 
is greatest in the most humid seasons of the year ; it accounts to 
us for the greater absolute size of rain-drops in the hottest 
months and near the ground, as compared with those in the 
winter and on mountains ; finally, it is almost an inevitable 
consequence from what is known of the gradation of temperature 
in the atmosphere, that some effect of this kind must necessarily 
take place Lastly, I cannot forbear remarking that 
this hypothesis of augmentation of size of the elementary drops 
agrees with the result that the increase of quantity of rain for 
equal lengths of descent is greatest near the ground ; for whether 
the augmentation of each drop be in proportion to its surface or 
its bulk, the consequence must be an increasing rate of 
augmentation of its quantity as it approaches the ground.”^ 
It seems unfortunate that a theory so plausible should not 
be also true, but, without spending time in detailed criticism, it 
may be stated that Sir John Herschel, in his Essay on 
Meteorology, showed by calculation that under no conceivable 
circumstances could the condensation of vapour upon the falling 
drops of rain amount to more than a minute fraction of the 
difference actually observed. This explanation must therefore 
be given up. 
It has been generally assumed that no other method is 
possible by which drops of rain can increase in size as they fall 
but that by condensation of vapour. Asa matter of theory this 
^ British Association Bejoort, 1833. 
