Water for Drinking. 215
been supplied by rain, dew, or fog-damp, falling on the face of the land and sinking into it. But the subsoil and rocks below, are far from being of a uniform character : they are full of layers of every imaginable degree of sponginess. Strata of clay wholly impenetrable by water, often divide beds of gravel that imbibe it freely. There are also cracks that make continuous channels and dislocations that cause them to end abruptly ; and there are rents, filled with various materials, that may either give a free passage or entirely bar the underground course of water. Hence, when water has sunk into the earth, it does not by any means soak through it in an equable degree. It is an easier matter for it to ooze many miles, along a layer of gravel, than to penetrate six inches into a layer of clay that may bound the gravel. Therefore, whenever a porous earth or a fissured rock crops out to the light of day, there is, in ignorance of all other facts, some chance of a spring being discovered in the lowest part of its outcrop. A favourable condition for the existence of a large and permanent fountain, is where a porous stratum spreads over a broad area at a high level, and is prolonged, by a gradually narrowing course, to an outlet at a lower one. The broad upper part of the stratum catches plenty of water during the wet season, which sinks into its depths as into a reservoir, and oozes out in a regular stream at its lower outlet. A fissured rock makes a still easier channel for the water.
As examples of ordinary cases of fountains, we will take those represented in the following figures. Fig, 1 is a moun
tain. Fig. 2 is a model, made to explain more clearly the conditions represented in fig. 1. It will be observed that there is a ravine, R, in front; a line of fault, L, 1, N, on its left side,