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



tion of the current ; and, judging from facts of observation, it seems to be at some 

 point less than 40 feet from the ground that a great and sudden coalescence of 

 the small globules of water takes place, — the effect, probably, of the current being 

 first retarded and then forced into a lateral direction, and of the irregular 

 mingling together of the particles of water to which this gives rise. 



The hypothesis next assumes as true the following important proposition, — 

 namely, that while a rain-gauge, placed at some height above the ground, receives 

 all drops having a sensible magnitude, it receives none of those globules which 

 are so minute that the velocity of their descent, though partly the effect of 

 gravity, is chiefly due to the motion of the downward current of air on which 

 they float. These very minute globules, it assumes, are borne by the current 

 past the mouth of the gauge, and continue to descend until, by the coalescence of 

 a great number of them, a drop is formed large enough to fall into a lower gauge 

 if placed in its path. 



This essential point of the hypothesis may be illustrated by what is observed 

 when wind blows through a room having two windows opposite to each other. 

 In such a case the leeward Avindow is no sooner closed, than the breeze, if gentle, 

 is scarcely felt within the room, though the other window remain open ; and the 

 obvious reason is, that the air inside the room, being now supported at all 

 points, resists the entrance of any more air by the open window. In a similar 

 manner, it is conceived, the slow downward current of air and floating mois- 

 ture fails, as the hypothesis assumes, to find entrance into the mouth of 

 a rain-gauge, and is forced to turn aside and to continue its descent towards the 

 ground. 



But here the question may occur, what will be the result if this downward 

 current be combined with a current of wind at right angles to it ? One effect 

 doubtless will be, to accelerate that process of coalescence which, it has been 

 assumed, takes place among the minute globules of water in the downward 

 current. And this more rapid coalescence would evidently tend to increase the 

 indication of the upper rain-gauge, and thereby to equalise it with that of the 

 lower gauge, were it not that this tendency is more than counterbalanced by the 

 wind uniting with the downward current of air, to turn aside from the mouth of 

 the gauge, not only the very smallest of the globules of water, as during a calm, 

 but also the smaller of those globules which, under ordinary circumstances, would 

 have fallen into the gauge. It appears therefore that, as respects its ultimate 

 effect, the wind increases the difference between the indications of the upper and 

 lower rain-gauges, and this is in accordance with observation. 



But besides the wind, which, it has been shown, increases the ordinary 

 effect of the downward current, there are a few causes which seem to lessen or 

 mask its influence. Thus, it is well known that, on some rare occasions, the 

 quantity of rain in the upper gauge equals, and even exceeds, the quantity in the 



