244 On Draining and Watering.(April,

face of this sheet of clay, between the lips of these rocks, a perpetual dribbling of water oozes quite round the grotto, like a shower from a weeping rock. Such sheets of water having been often observed to slide between the strata of the earth almost horizontally, like the horizontal joints of a stone-wall, have, it is supposed, given the name of wall-springs to them, to distinguish them from pipe-springs, or such as burst out in a single mill.

Thus this thin sheet of clay prevents the water from sinking into the lower stratum of sand-stone; and produces other copious springs, which are collected at about half'a mile’s dis- tance, and conveyed by leaden pipes to the cathedral close of Litchfield, which is thus supplied with water of uncom- mon purity, which contains no calcareous earth, owing to its passing through siliceous sand over a stratum of clay, and which would be a treasure to the paper-mill or the bleach- yard.

One other circumstance in the present conformation of the earth is necessary to be mentioned; which is, that at the time when the mountains were raised all over the world by deep volcanoes, or by central fires, some parts of the summits of many of them, and of their steeper sides, rolled down again into the new formed vallies. And secondly, that since that remote time the recrements of vegetable and animal bodies have continually been washed down from the eminences by showers, and have contributed gradually to accumulate in the vallies, and to form the plains, which exist on the sides of rivers. This appears from the tin ores found in the vallies in Cornwall, in loose pieces, similar to those in the proximate mountains; and fromthe black carbonic soil, or morass-turf, found in most vallies.

From these clear ideas of the strata of the earth, and of the streams of water, which slide between them, and form what are termed wall-springs, it is easy to conceive, that the best method of preventing the vallies at the bottom of hills from being too moist must be by cuttinga long horizontal ditch in- to the side of the mountain to intercept the water, just before the level land of the valley commences; and thus to carry away the water before it comes upon the plain beneath.

For this purpose at the foot of the nll, where the plain which is too moist, commences, some auger-holes should be bored to find the depth of the springs, that 1s to find the thick- ness of the upper stratum of the soil. If this be only four or six feet, an horizontal ditch should be cut along the bottom of the mountain to intercept the water; which must then be carried away by one or more other ditches opening into this, pi raga the water so collected into the neighbouring rivulet.