46 
questions of underground water-supply. Reasoning back on these 
grounds, from effects to causes, we might safely argue that over 
any area where the mileage of watercourses happens to be relatively 
large, there most of the rain-water runs off, and but a small part 
of it sinks into the rock ; while in the districts where the mileage 
of water courses is small in relation to the area, there a corre- 
spondingly-large percentage of the total rainfall percolates into-the 
rock, and lies beneath the surface instead of upon it. 
Water descends into rocks through the action of two causes; of 
these gravity is the more important, as the mere weight of the 
water carries it downward along joints or fissures or any line of 
weakness on the way. The other cause is capillary attraction, 
which helps stones of all kinds to absorb water in larger or smaller 
quantities. A widely-jointed rock will imbibe large quantities of 
water through the action of gravity, aided, of course, by capillary 
action ; while a stone of open texture will drink in as much as a 
gallon and a half of water per cubic foot of stone. Chalk will 
absorb as much as two gallons per cubic foot, and will transmit to 
the strata adjoining a large proportion of the water thus taken in. 
Rocks vary much in respect to the proportion of water they are 
able to transmit as compared with the quantity retained within the 
stone as quarry water. But in open-grained freestones the quantity 
that may be thus transmitted is usually very high, so that in these, 
if little or no clayey matter be present, there is a slow but steady 
descent of the rain-water from the surface to-all parts of the rock 
below, at least as far as the plane of saturation. As this plane is 
lowered by pumping, by the emission of water in the form of 
springs, or by any other cause, the supply is balanced by the down- 
ward percolation from above. And it should be noted here that 
slow-filtering through freestone rock, especially if that rock contain 
much irony matter, tends to free the water from any impurities, 
organic or inorganic. It is to this cause that the Manchester 
supply from wells owes its exceptionally good quality. Even the 
water from the canals becomes purified in its underground course, 
and is thereby rendered potable and suitable for domestic use in 
general, 
