WHY DO SPRINGS AND WELLS OVERFLOW? 403 



earth. He also refers to springs on tlie coast which gush out from 

 vertical cliffs of chalky limeBtone, which in the same way increase 

 largely in strength immediately after rain. 



That artesian wells are not sensibly affected by particular rain- 

 storms is no proof that they are not ultimately supplied by rains, but 

 only shows that the quantity of water furnished by the wells is ex- 

 ceedingly 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 w^ould 

 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 trick- 

 ling 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 valu- 

 able information, Mr. J. T. Fanning says of such a geological forma- 

 tion 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 compax-atively 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 ex- 

 ceeds 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 Cyclopaedia," 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 moun- 

 tain-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." " L^npracticed 

 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 



