Underlying all soils, at greater or less depth, is found some bed of rock, or clay, impervious to water, usually at but few feet below the surface--the descending water meeting with obstacles to its regular descent. The tendency of the rain-water which falls upon the earth, is to sink directly downward by gravitation. Turned aside, however, by the many obstacles referred to, it often passes obliquely, or almost horizontally, through the soil. The drop which falls upon the hill-top sinks, perhaps, a few inches, meets with a bed of clay, glides along upon it for many days, and is at last borne out to be drunk up by the sun on some far-off slope; another, falling upon the sand-plain, sinks at once to the "water-line," or line of level water, which rests on clay beneath, and, slowly creeping along, helps to form a swamp or bog in the valley. Sometimes, the rain which falls upon the high land is collected together by fissures in the rocks, or by seams or ruptures in the impervious strata below the surface, and finds vent in a gushing spring on the hill-side. We feel confident that no better illustration of the theory of springs, as connected with our subject, can be found, than that of Mr. Girdwood, in the Cyclopedia of Agriculture--a work from which we quote the more liberally, because it is very expensive and rare in America: "When rain falls on a tract of country, part of it flows over the surface, and makes its escape by the numerous natural and artificial courses which may exist, while another portion is absorbed by the soil and the porous strata which lie under it. "Let the following diagram represent such a tract of country, and let the dark portions represent clay or other impervious strata, while the lighter portions represent layers of gravel, sand, or chalk, permitting a free passage to water. [Illustration: Fig. 5.]