5 
that the constituent particles of cloud may be formed in abundance in the 
lower air during rain, being only prevented from assuming the visible form 
of cloud by reason of their absorption going on pari passu with their 
production. Let some influence be discovered in operation during rain 
which shall effect a continuous condensation of vapour from the lower 
strata of the air, and we shall have found a source whence the falling rain- 
drops may be augmented to an indefinite extent. 
Such an influence I believe to be exerted by the earth itself, acting as 
a cold body towards the comparatively warm and almost saturated air that 
rests upon it. We know that the air during rain is heavily charged with 
vapour. We know also that the temperature of the earth's surface is often 
many degrees below that of the air. We see that when circumstances are 
specially favourable to the cooling of the ground by radiation, fog forms 
even in dry weather. In rainy weather, it is true, the circumstances are 
not favourable to such a cooling process. Still, considering the humidity 
that then prevails, and remembering that the earth, if previously cooled by 
radiation, cannot immediately acquire the temperature of the air, we may 
not unreasonably infer that during rain a process of condensation goes on 
in the neighbourhood of the earth's surface, the result of which, if it can- 
not be called actual cloud, may be described as potential cloud — a term by 
which we may conveniently express the idea, that if the process of 
absorption by the falling rain-drops were intermitted, while the process of 
condensation still went on, visible cloud would be formed. 
It remains to apply such tests as are available to prove the soundness or 
unsoundness of the view now advanced. 
In the first place, it is clear that, if the earth be the cause, the effect 
should be marked in proportion to the proximity to the earth. Fortunately, 
the materials are abundant for the application of this test. Subjoined are 
two tables giving the results of an analysis of two series of observations, 
the one series by Colonel Ward at Calne, the other by Mr. Chrimes at 
Eotherham. The observations themselves are published in Symons's 
British Rainfall fox 1867 and 1870. They were conducted in both cases 
with extreme care, and in the Calne observations the ground-level gauge 
was sunk in a pit, in order to prevent error by splashing from the 
surrounding soil. The tables show in each case the rate of decrease per 
cent, in the amount of rain collected at each elevation, calculated on the 
amount collected at the elevation next below it. 
