478 ARTESIAN WELLS. 
to admit a considerable stream.* But in general, the sheets 
and currents of water reached by deep boring appear to be 
primarily due to infiltration from highlands where the water 
is first collected in superficial or subterranean reservoirs. By 
means of channels conforming to the dip of the strata, these 
reservoirs communicate with the lower basins, and exert upon 
them a fluid pressure sufficient to raise a column to the sur- 
face, whenever an orifice is opened.t| The water delivered by 
an artesian well is, therefore, often derived from distant 
sources, and may be wholly unaffected by geographical or 
meteorological changes in its immediate neighborhood, while 
the same changes may quite dry up common wells and springs 
which are fed only by the local infiltration of their own nar- 
row basins. 
In most cases, artesian wells have been bored for purely eco- 
nomical or industrial purposes, such as to obtain good water for 
domestic use or for driving light machinery, to reach saline or 
other mineral springs, and recently, in America, to open fount- 
ains of petroleum or rock-oil. The geographical and geological 
effects of such abstraction of fluids from the bowels of the earth 
are too remote and uncertain to be here noticed ; { but artesian 
* CuarLES Martins, Le Sahara, in Revue des Deux Mondes, Sept. 1, 1864, 
p. 619; SrorPaNnt, Corso di Geologia, i., 281; Dusor, Die Sahara, Basel, 1871, 
pp. 59, 51. 
+ It is conceivable that in shallow subterranean basins superincumbent 
mineral strata may rest upon the water and be partly supported by it. In 
such case the weight of such strata would be an additional, if not the sole, 
cause of the ascent of the water through the tubes of artesian wells. 
The ascent of petroleum in the artesian oil-wells in Pennsylvania, and, in 
many cases, of salt-water in similar tubes, can hardly be ascribed to hydro- 
static pressure, and there is much difficulty in accounting for the rise of water 
in artesian wells in many parts of the African desert on that principle. Perhaps 
the elasticity of gases, which probably aids in forcing up petroleum and saline 
waters, may be, not unfrequently, an agency in causing the flow of water in 
common artesian borings. It is said that artesian wells lately bored in Chi- 
cago, some to the depth of 1,600 feet, raise water to the height of 100 feet 
above the surface. What is the source of the pressure ? 
+ Many more or less probable conjectures have been made on this subject, 
but thus far I am not aware that any of the apprehended results have been 
