4 
But is it right to exclude the effect of condensation ? May it not be that 
the rain- drops falling from the higher, and therefore colder, regions of the 
atmosphere, act as cold objects towards the comparatively warm and very 
moist air through which they pass near the earth, thus condensing watery 
vapour upon their own surface and becoming augmented in volume ? This 
idea was put forward many years ago, and was accepted as an adequate 
explanation, until it was pointed out by Sir John Herschel that the increase 
due to such a cause admitted of exact calculation. It could not indeed be 
predicated of any particular instance what the increase would be, but it 
might be shown very positively what it could not be. Herschel took as the 
subject of his calculation the observations of Phillips, and purposely 
adopting suppositions as to temperature and other matters extravagantly 
favourable to the condensation theory, he yet found that it would only 
account for one-seventeenth part of the difference actually observed. The 
true proportion must be very much less than this. 
Thus then the matter stands at present. A notable excess of rain is 
observed to fall near the ground as compared with a higher level. All 
attempts to prove this excess illusory have failed. We are bound therefore 
to believe it real. We are bound to believe that an important proportion 
of the rain that falls on the earth is generated below the level of the tops 
of our houses. Yet the only plausible theory hitherto devised to account 
for this generation is found to be untenable. 
Let us meet the difficulty boldly. Eain is formed near the earth. Theory 
forbids us to believe that it is formed in any considerable degree by the 
direct condensation of vapour upon the cold drops from above. Then it 
must be formed by the incorporation of minute watery particles, the con- 
densation of which has been already effected through the operation of some i 
independent influence. And here it is necessary to insist on the distinc- ; 
tion between vapour and cloud. The vapour of water is aeriform and 
invisible. Cloud on the contrary is composed of liquid water in a finely 
divided state. The quantity of vapour that can be condensed under given 
circumstances is strictly limited, and admits of exact calculation. The 
quantity of cloud that can be absorbed by drops of rain falling through itj 
is subject to no such limitation. If therefore we may only suppose th^ 
presence of a thin cloud enveloping the earth during rain, the problem is 
half solved. But it may be fairly objected that we are not at liberty t<! 
make this supposition. If cloud were present it would be visible as cloud 
whereas it is only on rare occasions, in non-mountainous districts, tha 
visible cloud rests on the earth, even during rain. Yet it is couceivaW 
