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



quantity of vapour. But it is evident that this cause, though not uninfluential, 

 is totally inadequate to account for so great a difference. Admitting a given 

 weight of rain to arrive at 213 feet from the ground, with the temperature of 

 the region at which it was formed unaltered, and supposing it to acquire, in the 

 remaining 213 feet, the full temperature of the air (both of them extreme and even 

 extravagant suppositions), admitting, too (though hardly less extravagant), the 

 mean height of the formation of rain to be 12,000 feet, it would bring down with 

 it a cold of 40° Fahr., which would condense (whether on the drops or in satu- 



40 1 

 rated air, if diffused through it) only ^^^ = ^ 7 = 0-42 of its weight, = one-seven- 

 teenth of the quantity to be accounted for." 



Although this demonstration does not really admit of being strengthened, yet, 

 as showing how a similar conclusion was reached in a different manner, I beg to 

 refer to a short paper in the 20th volume of the " Transactions of the Royal 

 Society of Edinburgh," entitled, " On the Weight of Aqueous Vapour which is 

 Condensed on a Cold Surface under given conditions," in which I have endea- 

 voured to prove experimentally that, taking the observations at York during the 

 three winter months of the years 1832-33, 1833-34, 1834-35, the increment 

 which the rain received in falling between the level of the top of the Museum 

 and the ground, a height of 44 feet, was above GOO times greater than it would 

 have been if the condensation of aqueous vapour by cold had been the only cause 

 in operation. 



For these reasons, therefore, it seems necessary to reject this explanation, 

 though one of such likelihood as to have suggested itself, independently, to Dr 

 Franklix, M. Boisgiraud, and Professor Phillips. 



The eminent meteorologist Luke Howard proposed* an explanation which, 

 however, seems to differ from the one just considered chiefly in that it supposes 

 the cold, on which the condensation of vapour depends, to originate, in some unex- 

 plained way, in the atmosphere itself, instead of being brought down by the rain 

 from the upper regions. 



There is another mode of accounting for the diflBculty under consideration, 

 which has been advocated by Mr JevonsI and other writers. | According to this 

 theory, the deficiency of rain in the upper gauge is produced by wind — the gauge 

 itself, or the building on which it stands, giving rise to eddies which partially 

 obstruct the entrance of the rain into the mouth of the gauge. 



That wind may affect the indications of a rain-gauge, has been proved by the 

 interesting experiments of Professor A. D. Bache of Philadelphia.^ Having 

 placed gauges at the four angles of a high square tower, at a height of ten inches 



* Report of British Association for 1834, p. 563. 



f Philosophical Magazine for December 1861, p. 421. 



J Dr Stark in Transactions of the Royal Scottish Society of Arts, vol. v. p. 66. 



§ Report of British Association for 1838, Transactions of the Sections, p. 25. 



