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III. — On a Difficulty in the Theory of Rain. By James Dalmahoy, Esq. 



(Read 7th April 1862.) 



Nearly a hundred years ago,* Dr Heberden of London made the following 

 experiment : — Having prepared three exactly similar rain-gauges, he placed one 

 of them on the roof of Westminster Abbey, another on the roof of a neighbouring 

 house, but at a much lower level, and the third in the garden of the same house. 

 At the end of twelve months he found that the gauge on the roof of the Abbey 

 had received 12099 inches of rain, the gauge on the neighbouring house-top 

 18-139 inches, and the gauge on the ground 22*608 inches. 



This paradoxical result required, of course, to be confirmed by other observers, 

 and in other localities ; and the similar results obtained by Dobson, Dalton, 

 Howard, and especially by Arago at the Paris Observatory, and by Phillips at 

 York, have amply supplied all that Avas wanting in this respect. 



It may be noted in passing, as a curious fact, that in Dr James Hutton's 

 " Theory of Rain" there is no allusion to Dr Heberden's observations, though 

 these were published in the "Philosophical Transactions"! fourteen years before 

 Dr Hutton's Theory was read to the Royal Society of Edinburgh. J 



Of the attempts which have been made to reconcile Dr Heberden's observa- 

 tions with facts and principles already established, that of Dr Franklin is the 

 most plausible, and it has been very generally accepted as the true explanation. 

 At first sight, indeed, it seems capable of explaining every difficulty ; and it is 

 only when more carefully examined, and especially when tested quantitatively 

 by actual results, that its inadequacy becomes apparent. 



As, however, many may be disposed to question this conclusion, I am glad to 

 be able to rest the proof of it on the following quotation from Sir John Herschel's 

 treatise on Meteorology. § 



Having alluded to the fact that, during the year 1838-34, the quantity of 

 rain received on the top of York Minster, at the height of 213 feet, bore to that 

 received on the neighbouring ground the ratio of 1 : 1-706, the learned author 

 proceeds as follows : — " The usual account given of this phenomenon (Koemtz) 

 is, that rain falling from a high level, and therefore colder than air at the surface 

 of the ground, arriving in an atmosphere nearly or quite saturated with moisture, 

 condenses on itself, or causes the condensation, in the chilled air, of an additional 



* 1766-67. t 17J0. + 1784. 



§ Encyclopsedia Britannica, 8th edition, article Meteorologi/, par. 109. 



VOL. XXIII. PART I. I-I 



