18 
Arterial Drainage 
whilst the latter takes places in flat spongy moorland or culti- 
vated ground.* 
Professor Rankine gives the following figures as a guide to 
the proportion borne by the available to the total rainfall in 
different districts : — 
Proportion of available 
to total Eaiufall. 
Steep surfaces of granite, gneiss, and 
slate nearly 1 
Moorland and hilly pasture, from . . 0 • 8 to 0 • 6 
Flat cultivated country, from . . , 0*5to0'4 
Chalk 0 
Deep-seated springs and wells give from 0 • 3 to 0 * 4 of the total 
rainfall.! 
Mr. Bailey Denton puts the mean total discharge of the rivers 
at their outfalls in floods and freshets, from rainfall which has 
found its way over the surface of the ground without entering 
it, in the north and west of England, at 20 inches ; and for the 
midland, southern, and eastern districts at 6 inches ; or a mean 
for the whole country of 15 inches ; while the proportion of 
rain required to maintain the natural flow of our rivers during 
the summer and dry weather periods of the year is about one- 
eighth of the average mean rainfall, or 4 inches over the whole 
of the river watersheds.^ 
In calculating the proportion of rainfall which any given 
stream will discharge, there must be taken into consideration, 
besides the disturbing causes already alluded to, the nature of 
the soil over which the river passes. A very large proportion 
of the water is in some cases abstracted from the stream by 
permeable strata, which it encounters on its course. Thus, 
the River Churn, a tributary of the Thames, which derives its 
source from the flow of strong springs in the Cotswold Hills, 
after running through the Lias clay for the first part of its'course, 
comes to the Oolitic strata, when the quantity flowing down 
the channel, instead of increasing, suddenly decreases. From 
gaugings taken by Mr. Simpson, C.E., in the dry period of 
the autumn of 1859, the quantity was found to decrease from 
320 cubic feet per minute at 5J miles from the source to 10 cubic 
feet at 14 miles. The water in the intervening space percolated 
through the fissures and fractures of the rocky bed, and through 
the porous strata of the fish- and mill-ponds. After thi^ it 
began again gradually to increase in volume until it attained 
* " Water Supply of Paisley," ' Trans. Instit. Civil Engineers,' vol. xxxi. 
t Rankine's ' Manual of Civil Engineering.' Griffin and Co., 1871. 
j ' Tlie Storage of Water.' By J. Bailoy Denton. Spon and Co., 1874. 
