10 Mr Dalmahoy on the Cause of the Dimitiution 



On the Cause of the Diminution of the fall of Rain as the 

 height above the ground increases. By James Dalmahoy, 

 Esq. Communicated by the Author. 



The fact that a rain-gauge, placed in an elevated and de- 

 tached situation, receives less rain than an exactly similar 

 gauge near the ground, seems to have been first observed 

 about seventy-five years ago by Dr Heberden ; and has since 

 been abundantly confirmed, especially by the observations of 

 Dr Dalton, by a long continued series at the Paris Observa- 

 tory, and by those of Professor Phillips at York. 



The most generally received explanation of this curious 

 fact is that which was originally proposed by Dr Franklin, 

 and subsequently, but independently, by Professor Phillips. 



This hypothesis is attended with several difficulties, one of 

 which was suggested by Dr Franklin himself.* But perhaps 

 the greatest objection to it is founded upon a fact well esta- 

 blished by the observations at York, and exhibited in the 

 following small table ; — t 



Ypnv Depth of rain 



^^*^- in inches. 



Increment which the rain received while \ 1832-33 4.467 



falling between the upper and middle > 1833-34 3.841 



rain-gauge,through a height of 169 feet,) 1834-35 4.889 



Total, 13.197 



. Increment which the rain received while "^ looo oo o cm 



falhng between the middle ram-gauge, y ^ggo 34 3 qq^ 



and the gauge on the ground, through C ^gg^^g^ ^ 

 a height of 44 feet. -^ 



Total, 13.261 



It appears from this table that the increase which the rain 

 received during the last 44 feet of its descent, was almost 

 exactly equal to that which it received during the preceding 



* Manchester Memoirs (old series) vol. ii., Letter from Dr Franklin 

 to Dr Percival. 



t Reports of the British Association for 1833, 1834, 1835. 



