56 
The ordinary summer flow may exceed this probably to the 
extent of 50 to 100 per cent., making it say 30 or 40 millions of 
gallons per diem ordinary summer flow; 323 sq. miles, at 12 
cubic feet per sq.' mile per minute, would give 34,884,000 
“gallons per diem, and about 1,046 millions of gallons per diem 
e excessive flood flow, if taken to be 30 times as much, a volume 
of water that it would seem there should be no great difficulty 
in carrying off by the simple removal of obstructions and a 
better regulated passage for the flood waters. 
As to the flood flow of the Thames, and indeed of all rivers, 
that materially depends upon the proportion of rainfall which 
finds its way into the river, which varies with the nature of the 
strata of the basin and the season of the year. The height of 
a river is also influenced by the period occupied by the rainfall 
in reaching the river. 
Mr Vernon Harcourt, in an admirable paper lately read 
by him to the Institution of Civil Engineers on the Seine, 
the drainage area of which, with its tributaries, is about six 
times that of the Thames, but which river bears much resem- 
blance to the Thames and its tributaries in Geological character 
through a large portion of the districts which they respectively 
traverse, points out that although the rainfall over the Seine 
basin above Paris is greater in summer than in winter in 
the proportion of 21 to 17 about, yet the flow of the river at 
Paris is nearly double through the winter months what it is 
through the summer months, or adopting the carefully elabo- 
rated returns of the French Engineers, “the winter months 
discharge through the river 43 per cent. of the rainfall, whilst 
the summer months discharge but 17 per cent. of the larger 
rainfall.” 
This is due to percolation, or rather infiltration and evapo- 
- ration, being so much more operative in summer than in winter. 
All the floods in the Seine between May Ist, 1872, and April 
30th, 1884, Mr Harcourr finds to have occurred during the 
cold season. 
I have examined the valuable returns of the daily volume of 
water flowing down the Thames at Thames Ditton, a mile and 
