32 MR DALMAHOY ON A DIFFICULTY IN THE THEORY OF RAIN. 



current of air, and dispersed thi'ough a very large space, is assumed to be so rare 

 as not to affect the transparency of the atmosphere ; and it is the constant float- 

 ing down of this medium in a current of air which, according to the hypothesis, 

 is the principal and almost sole cause of the phenomenon to be accounted for. 



But it will be necessary, at this point, to answer a question which may 

 naturally be anticipated, — namely, whether there be any proof, from theory or 

 observation, that rain is actually accompanied by a downward current of air 

 and floating moisture. 



Theoretically, two causes may be assigned for such a current. The 

 first is the continual displacement of the air by the downward motion of the 

 drops of rain ; for, the effect of bodies in rapid motion to draw after them any 

 light matter in their neighbourhood, such as air or smoke, is familiar to every- 

 body ; and that the drops of rain should have this effect will not seem impro- 

 bable, when it is remembered that the largest of them may attain a velocity of 

 nearly 400 inches per second. 



The second cause of the downward current of air which theory suggests, is 

 the cold which rain brings with it from the upper regions. This must render the 

 air inside the limits within which the rain is falling heavier than the air outside 

 those limits, and thus co-operate, in an obvious way, with the former cause. 



But in order to prove that theory is in this case borne out by observation, I 

 adduce the two following quotations from writers of unquestionable authority. 



Professor Phillips, in his first Report* on the observations at York, makes 

 the following remark : — " I have noticed in several instances the fact, that the 

 wind which accompanies the fall of rain takes the line of the rain-drops them- 

 selves ; and on the Minster, in particular, this was very strikingly illustrated 

 when, with my friends Mr Jonathan Gray and Mr William Gray, junior, I 

 watched the progress of a storm for thirty miles down the vale of York. The wind 

 was insensible, except during the fall of rain, and then it came downward with 

 the drops." 



This testimony establishes the fact, that rain is sometimes observed to be 

 accompanied by a downward current of air so strong as to ]]e described as a 

 " wind." It also records two other facts, which have a bearing on the hypothesis; 

 the first of these is, that on the occasion on which the downward wind particu- 

 larly attracted notice, the observers were not standing on the ground, but on the 

 top of the Minster, at the height of 213 feet ; and the second fact is, that the rain 

 and downward wind began and ceased simultaneously, which affords a strong 

 presumption in favour of their being connected as cause and effect. 



The other quotation which I have to adduce is from Emerson Tennent's 

 " Ceylon,"f and is as follows : — " The first fall of rain was preceded by a down- 



* Report of British Association for 1833, p. 404. f Vol. i. p. 69. 



