
- Parr IL. Szor. ii. §3.] FLOW OF RIVERS, 361 
a part of the regular system of water circulation over the land, they 
do not represent the ordinary proportions between rainfall and river 
discharge in such a climate as that of Britain, where the rainfall 
is spread more or less equally throughout the year. According to 
Beardmore’s table,’ the Thames at Staines has a mean annual discharge 
of 32°40 cubic inches per minute per square mile, equal to a depth 
of 7°31 inches of rainfall run off, or less than a third of the total 
rainfall. The most carefully collected data at present available are 
probably those given by Humphreys and Abbot for the basin of the 
Mississippi and its tributaries as shown in the subjoined table :—* 
Ratio of Drainage 
to Rainfall. 
Ohio River . ; " : : : ‘ °24 
Missouri River , ; 
Upper Mississippi River. 
Small tributaries : 
Arkansas and White River 
Red River . 5 ; 2 : : 
Yazoo River . : 5 . ; 3 
St. Francis River . : : 5 i 
Entire Mississippi, exclusive of Red River 
SSS 15S) SS Sr) 
bt 
Or 
In the Mississippi basin one fourth of the rainfall is thus discharged 
into the sea. The Elbe, from the beginning of July 1871 to the end 
of June 1872, was estimated to carry off at most a quarter of the 
rainfall from Bohemia.* The Seine at Paris appears to carry off 
about a third of the rainfall. In Great Britain from a fourth to a 
third part of the rainfall is perhaps carried out to sea by streams.‘ 
In comparing also the discharges of different rivers regard should 
be paid to the influence of geological structure, and particularly of 
the permeability or impermeability of the rocks as regulating the 
supply of water to the rivers. Thus the Thames, from a catchment 
basin of 3670 square miles and with a rainfall of 27 inches, has a 
mean annual discharge at Kingston of 1250 millions of gallons a 
day, and rather more than 688 millions of gallons in summer. The 
Severn, on the other hand, which gathers its supplies mainly from 
the hard, impervious slate rocks of Wales, has a drainage area above 
Gloucester of 3890 square miles, with an average rainfall of probably 
not less than 40 inches. Yet its summer discharge does not amount 
to 298 millions of gallons,'and its minimum sinks as low as 100 
millions of gallons, while that of the Thames in the driest season 
never falls below 350 millions. In the one case the water is stored 
1 “Hydrology,” p. 201. 
2 «Physics and Hydraulics of the Mississippi River,” Washington, 1861, p. 136. 
3 Verhandl. Geol. Reichsanstalt, Vienna, 1876, p. 173. ; 
* In mountainous tracts having a large rainfall and a short descent to the sea, the 
proportion of water returned to the sea must be very much greater than this. Mr. 
Bateman’s observations for seven years in the Loch Katrine district gave a mean annual 
rainfall of 873 inches at the head of the lake, with an outflow equivalent to a depth of 
81-70 inches of rain removed from the drainage basin of 713 square miles. See a recent 
paper by Graeve on the quantity of water in German rivers, and on the relation between 
rainfall and discharge, Der Civil-Ingenieur, 1879, p. 591; Nature, xxiii. p. 94. 
