186 OBSERVATIONS ON THE FALL OF RAIN. 
mont exceeded sixty inches. Now supposing the 
same depth of rain to have fallen above the Tur- 
ton and Entwistle reservoir, during the year in 
which the forty inches were there collected, that 
quantity would be nearly two-thirds of that which 
fell. This agrees with the experience of the 
Bann reservoirs in average years, and with the 
observations of Mr. Thom, at the Greenock re- 
servoirs in 1828. 
This result is twice as much as the proportion 
Dr. Dalton assumes by his approximate calcula- 
tions, even with the addition to the depth of rain 
of five inches for dew. In those, however, there 
may be two principal sources of error: first, in 
the quantity of water which flows into the sea, 
and secondly, in the means of supply. 
First, as to the quantity of water flowing into 
the sea.— 
Dr. Dalton grounds his calculations on an 
assumption of Dr. Halley’s, as to the volume of 
the Thames, at Kingston; and, imagining that 
