Page:Popular Science Monthly Volume 16.djvu/423

Rh earth. He also refers to springs on the coast which gush out from vertical cliffs of chalky limestone, which in the same way increase largely in strength immediately after rain.

That artesian wells are not sensibly affected by particular rainstorms is no proof that they are not ultimately supplied by rains, but only shows that the quantity of water furnished by the wells is exceedingly small compared with the total quantity at any time in the layer of porous material tapped by the wells. Such layers, between two saucer-like formations of impermeable matter, would generally have some points of their outcrop at a lower level than others. At these low points of the outcrop natural springs would occur, which would have a flow more or less constant in proportion to the extent and height of the porous layer above them, and their flowing would continually tend to draw the level of the water in the porous layer down to their own altitude. Rains, falling on the exposed edges of the porous layer, would in great part be absorbed, and, gradually trickling through the pores, be slowly discharged by these natural springs. If an artesian well had its opening into the porous layer far below the lowest of these natural outlets, no ordinary rain would sensibly change the effective head of water that supplied it; but, if rains should cease entirely, the springs and the well would ultimately stop flowing. In a work on "Water-Supply Engineering," which contains much valuable information, Mr. J. T. Fanning says of such a geological formation as the common theory of artesian wells assumes, that when first discovered it "is invariably full to its lip or point of overflow. Its extent may be comparatively large, and its watershed comparatively small, yet it will be full, and many centuries may have elapsed since it was molded and first began to store the precious showers of heaven. A few drops accumulated from each of the thousand showers of each decade may have filled it to its brim many generations since; yet this is no evidence that it is inexhaustible. If the perennial draught exceeds the amount the storms give to its replenishment, it will surely cease, in time, to yield the surplus." (Compare with this the extract given above from "The American Cyclopædia," showing an annual sinking of two feet in the level of the water in the artesian wells near London.)

Mr. Green can not account for the flow of streams from the mountain-region of Pennsylvania and from Lake Chautauqua without the intervention of his "newly discovered force." He quotes approvingly a statement that "it is a wonder to the unpracticed observer where the water-supply of Chautauqua Lake comes from." "Unpracticed observer," indeed! But the practiced observer will tell you without hesitation that the water-supply comes from the clouds. Mr. Fanning (op. cit.) states, as the estimate from experiments, that "in the Eastern and Middle United States the evaporation from storage reservoirs, having an average depth of at least ten feet, will rarely exceed sixty