OBSERVATIONS ON THE FALL OF RAIN. 187 
Dr. Halley has overrated the quantity, takes only 
two-thirds of his result. Now we have seen that 
the ordinary measure of a river is very far from 
affording any criterion by which to judge of the 
volume of the floods, when much the greatest 
quantity of water which flows from the ground 
passes down the rivers; therefore the ordinary 
volume of the river Thames, whether right, as 
assumed by Dr. Halley, or as corrected to suit 
the impressions of Dr. Dalton, cannot be taken 
as a sufficiently accurate basis of calculation. 
There can be little doubt that the results would 
be far below the truth, and that the thirteen in- 
ches Dr. Dalton assumes as the depth of rain 
flowing into the sea, would require to be consi- 
derably increased. 
Secondly, as to the means of supply.— 
In endeavouring to ascertain the average quan- 
tity of rain, Dr. Dalton takes a mean of the 
observations which had at that time been kept 
throughout England. This, no doubt, gives a 
very correct idea of the rain falling in the gene- 
rally inhabited parts of the country; but it may 
be questioned if it forms a correct measure of the 
rain which forms the principal supply of rivers. 
