MR DALMAHOY ON A DIFFICULTY IN THE THEORY OF RAIN. . 33 



ward blast of cold air, accompanied by hailstones, which outstript the rain in its 

 descent." 



In this case the coldness of the hailstones and the velocity of their fall pro- 

 duced so rapid a current as to be described as a " downward blast ;" and it is 

 probably only when the hail or rain falls with unusual violence, as in this case, 

 that the vertical direction of the wind is perceptible to an observer at the level of 

 the ground. 



But if, for the reasons which have been adduced, it be granted that rain is 

 always accompanied by a downward current of air, of greater or less velocity, it 

 seems necessarily to follow that this current, originating, as it must do, in the 

 very region of cloud, will descend charged with minute particles of water. 



Having thus proved, as I hope, both by theory and observation, that rain is 

 accompanied by a downward current of air, mingled with minute globules of 

 water, I shall now endeavour to explain the twofold agency of such a current, in 

 causing the indications of equal rain-gauges to vary with their height above 

 ground. 



The first and most obvious way in which the downward current produces this 

 effect, is by filling the entire space through which the rain falls with a constantly 

 renewed supply of very minute globules of water. The rain- drops must, of course, 

 absorb as much of this watery medium as they come in contact with, each drop 

 growing in size during the whole time of its descent ; and the necessary result 

 must be, that if two equal gauges receive each an equal number of rain-drops, the 

 gauge nearest the ground will indicate the most rain. 



But, with reference to the process thus described, it will naturally be asked, 

 what becomes of the multitude of minute globules of water which are not absorbed 

 by the rain-drops ? The answer to this will, it is hoped, be found in the follow- 

 ing explanation of the second and less obvious way in which the downward cur- 

 rent affects the indications of rain-gauges placed at unequal heights above the 

 ground. 



Taking for granted, then, that during rain a slow current of air carrying minute 

 globules of water is continually descending from the region of cloud, I now assume 

 that these minute globules of water, in the course of their descent, often come 

 into contact with one another, and coalesce into drops of sensible magnitude ; and 

 this again leads to the inference that globules which, at a higher level, descended 

 chiefly by participating in the motion of the downward current of air, acquire, 

 after their coalescence, a velocity and momentum which enable them not only 

 to outstrip the current, but also to resist being carried out of their downward 

 direction by any lateral motion which may happen to be impressed upon the 

 current. 



This process of coalescence, it is conceived, becomes more and more rapid as 

 the resistance of the ground begins to tell on the velocity and downward direc- 



VOL. XXIII. PART I. I 



