Fifth book of lessons for the use of schools . sage, unite, and form a little 320 FIFTH BOOK. rivulet: this, in its progress, meets with other rivulets ofa similar description, and they pursue their coursetogether in the interior of the earth, till • they arestopped by some substance which they cannot pene-trate ; for though we have said that water under strongcompression penetrates the pores of gold, when actedupou by no other force than gravity, it cannot make itsway even through a stratum of clay. This species ofearth, though not remarkably dense, being of greattenacity, will not admit the
Fifth book of lessons for the use of schools . sage, unite, and form a little 320 FIFTH BOOK. rivulet: this, in its progress, meets with other rivulets ofa similar description, and they pursue their coursetogether in the interior of the earth, till • they arestopped by some substance which they cannot pene-trate ; for though we have said that water under strongcompression penetrates the pores of gold, when actedupou by no other force than gravity, it cannot make itsway even through a stratum of clay. This species ofearth, though not remarkably dense, being of greattenacity, will not admit the passage of water. When,therefore, it encounters any substance of this nature, itsprogress is stopped, and the pressure of the accumulatingwaters forms a bed, or reservoir. The next figure represents a section of the interior ofa hill or mountain. A is a body of water such as hasbeen described, which, when filled up as high as B (bythe continual accession of waters it receives from theducts or rivulets a, a, a, a,) finds a passage out of the. cavity; and, impelled by gravity, runs on, till it makesits way out of the ground at the side of the hill, andthere forms a spring, c. The spring, during its passagefrom B to c, rises occasionally, upon the same principlethat water rises in the spout of a tea-pot, but it cannotmount above the level of the reservoir, whence itissues; it must, therefore, find a passage to some part ofthe surface of the earth that is lower or nearer thecentre than the reservoir.—Water may thus be con-veyed to every part of a town, and even to the upperstories of the houses, provided that it be originallybrought from a height superior to any to which it isconveyed. SPRINGS, FOUNTAINS, ETC. 321 Ileservoir8 of water are seldom formed near theBummit of a hill, for in such elevated situations therecan scarcely be a sufficient number of rills to supplyone; and without a reservoir there can be no such situations, therefore, it is necessary to dig deepwe
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