212 DE rance: underground water-supply and river floods. 
available quantity for waterworks purposes is chiefly that stored, or pass- 
ing through circles Joints and fissures, whilst the remainder of the water 
held in the mass occurs between the particles, for the retention or ab- 
straction of which there is a constant struggle between capillarity 
and gravity, and though the water is received with great rapidity, it 
is parted with, with exceeding slowness. It is important to distinguish 
between the amount of storage capacity and the actual volume an- 
nually absorbed from rainfall, the one appertaining to water capital, 
the other to water revenue. 
When bands of permeable and impermeable rocks alternate, 
each porous band contains a separate sheet of water, which flows 
down the " dip planes" of the strata confined by the impermeable 
la3^ers above and below. Such water flows, with the " head," due to 
the difference of vertical level of the ''area of outcrop" to that of 
the area of discharge," less the frictional resistance of the frag- 
ments of the rock through which it passes. When the facilities for 
the discharge of a volume are less than the quantity capable of being 
received, the porous rock will be full up to the impermeable layer 
above, which is invariably the case when all outlet is stopped by 
faults throwing in impermeable strata, or by the dip carrying the 
strata beneath the sea-level. Such porous rocks may be regarded as 
underground conduits, the depth of which is the thickness of the 
bed, the width of which is the extent of the outcrop or horizontal 
strata of its bed, and the inclination of which is the dip of the strata. 
Wlien the outlet is blocked the saturation level remains unchanged, 
and unless water is artificially removed, so as to provide space for a 
fresh supply, no additional water can be added to the existing supply. 
From the investigations I made in 1878 as to the area of the 
various rocks in each river basin in England and Wales, which form 
the basis of the paper read before the Water Congress convened by 
the Society of Arts, at the instigation of their President, H.R.H. the 
Prince of Wales, I estimate the area in England and Wales, occupied 
by porous rocks, at 26,633 square miles; whilst the clays of the 
tertiary, gault, weald, oolite and lias, the marls of the trias and 
Permian, the shales of the carboniferous, occupy a further 19,308 
square miles, forming for the most part a " supra-pervious area," or 
