MR DALMAHOY ON A DIFFICULTY IN THE THEORY OF RAIN. 31 



above the parapet, he found that the gauges at the lee side of the tower received 

 more rain than those at the windward side ; but that, the same arrangement con- 

 tinuing, when gauges were also placed on poles at the height of six feet above the 

 parapet, there was observed scarcely any difference between their indications. 



The bearing which these results have on the present question will afterwards 

 be noticed ; but neither these, nor any similar facts which writers have adduced, 

 seem to meet the special difficulty to be explained — namely, that of three gauges, 

 at three different levels, and each variously placed as respects surrounding objects, 

 the lowest gauge always receives more rain than the middle one, and the middle 

 gauge more than the upper one. 



But that which seems an unanswerable objection to this mode of explanation. 

 is the fact that, when the upper and lower gauges have been inspected after a 

 perfect calm, and when the rain fell perpendicularly, the upper gauge was still 

 found to contain less rain than the lower one. This fact is recorded very ex- 

 pressly by Arago,* and also by Phillips.! 



While, therefore, it is denied, for the reasons now adduced, that the difficulty 

 under discussion can be accounted for by the effect of wind, it is not disputed that, 

 under certain circumstances, wind does modify the indications of a rain-gauge. 



Sir John Herschel concludes his notice of the attempts which have been made 

 to account for the phenomenon, in these words :+ — " The real cause is yet to seek, 

 and there is no more interesting problem which can fix the attention of the meteor- 

 ologist. Visible cloud rests on the soil at low altitudes above the sea-level but 

 rarely, and from such cloud only would it seem possible that so large an accession 

 of rain should arise." 



I was first led to think of this puzzling question a good many years ago, 

 and the result of my repeated attempts to find a solution of it is contained in the 

 following hypothesis, which, in spite of its many defects, I venture humbly to 

 submit to the consideration of those who take an interest in meteorology. 



The hypothesis begins by taking for granted the truth of Dr Hutton's 

 "Theory of Rain."§ It assumes that the spherules of water composing the 

 clouds from which rain proceeds are, at their first formation, so small, that the 

 terminal velocity of their descent is almost insensible. It further assumes that 

 these minute globules coalesce, at innumerable points, into drops of sensible 

 magnitude, and fall in the shape of rain ; while portions of the cloud, which do 

 not thus coalesce, are floated downward in a current of air, and fill the whole 

 space between the clouds and the earth with minute particles of water. 



This medium, consisting of cloud carded out, as it were, by the downward 



* (Euvres, tome xii. pp. 409, 416. 



I Report of British Association for 1833. See Transactions of Sections, pp. 403, 404. 

 Report for 1834, p. 561. 



J Article Meteorology, par. 109. 



§ Transactions of Royal Society of Edinburgh, vol. i. p. 41. 



