.. r. id. 256 On Draining and i atering.[April,

of water, are obvious to the workmen. In many situations hollow bricks, or ridge-tiies, or old pieces of plaster-floors, may be worth the additional expence of providing them.

There may nevertheless be found situations, where the first stratum of earth may be too thick to be easily penetrated; or where the water, condensed from the atmosphere on the summits of the hills, may slide between the second and third, or between the third and fourth strata, which form the sides of those hills, owing to a deficiency of so many of the strata at the summits ofthem; and hence that it may lie too deep to be easily arrested by a ditch, or by boring; and yet by its being dammed up by the materials, which form the level plain of the valley, may rise up through those materials } to the surface, and torm boggy e1 morassy ground.

In these situations the cominon unskilful method of drain- ing may be usefully employed; which consists in cutting many ditches four or six feet deep across the bog or morass; and covering them, so that the water may have no obstiuc- tion in passing along them; which may thus, as it rises from below, be in part collected and conveyed away; though less advantageously than where the springs can be interceped.

There are some situations, where.the water is conveyed be- neath the first stratum on a thin bedof clay over a porus sand- stone beneath it; as in the grotto at Litchfield above describ- ed. In these situations by boring many auger-holes, or by sinking wells, through the stratum of clay the water will pe- netrate the sand-stone beneath it; and either pass away by the porosity of this kind of stone, or by the cracks or joints which are always found in it; of which the horizontal joints were formed at the time of the production or accumulation of the sand beneath the sea, which was then formed in horizon- tal strata; but the vertical cracks were made at the time of its elevation by subterraneous fires. In these vertical fissures the i iti! ores of lead, ponderous earth, and calcareous spars, are found

in the Jime-stone rocks of Derbyshire; and those of tin, and quartz, in the granite rocks of Cornwall.

The knowledge of this part of geology concerning the formation of springs may be employed for many useful pur- poses; thus where the wall-springs, or water-conducting strata, jie so deep as not to be acceptable at a small expence; they generally exist between the second and third, or between the third and fourth strata; which rise into day higher on the adr jacent mountains than the first stratum; and hence, when they are bored into, the water will rise higher, than when it is found beneath the first stratum only; which generally becomes deficient on lower parts of the adjacents eminences of the country.