MR DALMAHOY ON A DIFFICULTY IN THE THEORY OF RAIN. 37 



question, and within the period of twenty-four hours, a current of very minute 

 globules of water — equivalent, in all, to a depth of half an inch of rain — was floated 

 downward past the level of the Minster gauge, without giving any indication of 

 its passage ; and that of this half inch, a portion, equivalent to 0*2 of an inch, 

 changed its form while descending between the levels of the Minster and Museum 

 gauges, — the larger part of this quantity combining to form drops of a size sufficient 

 to admit of their falling into the lower of these two gauges, and the remainder 

 coalescing with drops of rain already formed, and thereby rendering each drop 

 larger than when it passed the level of the Minster gauge. The effect of this coa- 

 lescence of the minute globules of water with each other, and with the larger rain- 

 drops was, according to the hypothesis, to raise the indication of the Museum gauge 

 to 0*7 of an inch, — that is, 0*2 of an inch above the indication of the Minster gauge. 



Again, it is assumed that the current, now containing a quantity of water 

 equivalent only to a depth of 0"3 of an inch, was carried past the level of the 

 Museum gauge without leaving any trace of its passage, and was by a similar 

 process of coalescence, but much more rapid than what took place at the higher 

 level, converted into drops which, owing to the cessation of the downward current 

 as it approached the ground, were now no longer, even the smallest of them, 

 carried past the mouth of the gauge placed there, but entering it, raised its indica- 

 tion to one inch,-^-that is, 03 of an inch above the indication of the Museum gauge. 



Having traced the agency of the downward current thus far, the next step 

 ought to be to ascertain the velocity of the current. But it is difficult to find any 

 data for making such an estimate ; for though it may be inferred, that the 

 velocity of the downward current of air which accompanies rain will have some 

 direct relation to the quantity of rain which falls in a given time, and to the 

 degree of cold which it brings down with it, yet, unfortunately, from not knowing 

 what that relation is in some actual instances, no use can be made of the general 

 principle. Since, however, it is necessary to arrive at some estimate on this point, 

 there are one or two considerations, connected with the hypothesis itself, which 

 seem to suggest a lower limit, at least, to the velocity of the downward current. 



In explanation of this, let it be assumed that (it being during winter) 3000 

 feet was the height of the clouds from which the one inch of rain fell in twenty- 

 four hours. Also let it be assumed that the diameter of the drops of rain was 

 uniformly one-tenth of an inch, which, as respects bulk, is greatly nearer the 

 lower than the higher of the two limits to their size which Professor Leslie has 

 assigned. Then, one inch of rain being supposed to fall in twenty-four hours, it 

 follows that the number of drops which would fall on any particular spot in the 

 same time would be fifteen,* and the interval of time between the consecutive 

 drops would be ninety-six minutes. In order, therefore, to keep the space between 

 the clouds and the earth replenished with the minute globules of water, the 



* Playfair's Geometry, Supp., B. III. Prop. xxi. 

 VOL. XXIII. PART L K 



