242 On Draining and Watering.[Apri, an adapted valve at the bottom to prevent the return of the water, and a valve at the extremity of each arm to prevent any. ingressof air aboye the current of‘the water as it flows out.

cccc, is a circular trough to receive the streams of water from C and D, to convey them where required,

ON DRAINING AND WATERING. By ee Darwin.

Lands in respect to the method of draining them may be divided into two situations; those which lie so high, that the water can descend from them, if it be properly co ollected and conducted; and those which lie so low as to command no } fall, some of which are even below the level of the sea.

In regard to the former it generally happens, that the wa- ters from the springs beneath the soil have not a free passage to the rivers in their vicinity; the nature of springs should therefore be previously understood. Many modern philoso-~ phers have endeavoured to shew, that all the continents and islands of the world, as well as thie| hills, which emboss their surface, have been raised out of the primeval ocean by snbter- raneous fires. This appears from the quantity of sea-shells, which form innumerable mountains; and from the fissures in the rocks, of which they consist; the quantity of volcanic productions all over the world; and the numerous remains of craters of volcanoes in Saar countries.

Hence the strata, which compose the sides of mountains, Jie-slanting downwards; and one or two or more of the ex- ternal strata not reaching tothe summit, when the mountain was raised up, the aascl or third stratum, or a more inferior di one, 1 there‘exposed to day.‘This may be well represented by forcibly thrusting a very blunt instrument through some folds of paper, a bur will be raised with the lowermost leaf standing Ve|) highest in the center of it. Or if at the original elevation of an extensive mountain the lowest stratum should not at first stand higher in the center of the summit, it would in time he- come so by some of the upper strata of the mountain being Srey washed away by rains into the valleys or riverse On this uppermost stratum, which is colder, as it is more ele- vated, the dews are condensed in large quanties; and sliding down pass under the first, or second, or third stratum, w hich

compose the sides of the re; and an HF form a morass below, or a weeping rock by oozing outin numerous places; or many

rf these je ss currents meeting together burst out in a more co- Sigua rill.

The immediate cause of springs consists therefore in the condensation of the atmospheric moisture, during the night principally, by the greater coldness of the summits of hills.