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



downward current ought to have a velocity of at least 3000 feet in 96 minutes, — 

 that is, about 7 inches per second, or fths of a mile per hour. 



In a table given by Dr Thomas Young,* it is stated that a wind blowing at 

 the rate of two miles an hour is just perceptible ; and as the downw^ard wind 

 recorded by Professor Phillips was distinctly felt, it may be concluded that its 

 velocity exceeded two miles an hour. Therefore, in adopting 7 inches per second, 

 or fths of a mile per hour, as the estimated velocity of the downward current in the 

 present case, there is, at least, the certainty that it is considerably less than a 

 velocity which observation has proved to be possible. 



And now, let it be imagined that a hollow prism reaches vertically from the 

 level of the Minster gauge to the ground, and that the area of its base is equal to 

 one square inch ; also let attention be directed only to that portion of the current 

 which may be supposed to descend through the prism ; — it is evident that in 

 the course of twenty-four hours, or 86,400 seconds, its volume will amount to 

 7x86,400 = 604,800 cubic inches. This volume consists of air and minute glo- 

 bules of water, the latter being, by supposition, equal in weight to half a cubic 

 inch of water = 126*23 grains. Hence, according to this estimate, each cubic 

 inch of the atmosphere between the levels of the Minster and Ground gauges, and 

 within the limits of the raining space, would contain, besides the aqueous vapour 



126-23 

 due to its temperature, only patotu) ~ 000021 gr. of condensed vapour, — i.e., less 



than half the quantity which would be requisite to saturate it, if dry, at zeroof Fahr. 



If, therefore, this estimate be at all near the truth, it seems to follow that 

 even such remarkable results as those of the winter observations at York may be 

 accounted for by the presence in the atmosphere of a quantity of condensed 

 vapour too small to give rise to the appearance of visible cloud. 



Before bringing the paper to a close, there remains to be noticed what, at first 

 sight, might seem to be a new source of diflBculty, namely, the fact that when the 

 elevation on which a rain-gauge is placed, instead of standing detached like a 

 house, or tower of a cathedral, forms part of a mountainous country, the ordinary 

 effect of elevation, in appearing to diminish the quantity of rain, is no longer 

 observed. Such a result, however, presents no real difficulty; for when the current 

 of air carrying minute globules of water which accompanies rain descends on an 

 elevated, but, at the same time, extended and uneven surface, the resistance 

 offered to its downward progress ought, according to the hypothesis, to produce 

 nearly the same effects as at the level of the ground. 



In concluding (his attempt to explain a long standing difficulty in the theory 

 of rain, I do so with an ■> nfeigned sense of the very imperfect manner in which 

 it has been executed, but, at the same time, with a good hope that it will be 

 found to be based on a true principle. 



* Lectures, vol. ii. p. 457. 



