Page:Popular Science Monthly Volume 16.djvu/84

 land on the earth, and then whence comes the flow to supply that? The mere statement of the case proves the existence of some force in nature other than hydrostatic pressure by which these vast bodies of water are driven to the surface. This hydrostatic pressure Professor Buckland thus illustrates and explains: "The portions of a water-logged, porous bed between two beds of clay may be illustrated by a tea-saucer placed within another tea-saucer, and having the narrow space between filled with sand and water. If a hole were drilled through the bottom of the upper saucer and a quill or small pipe fixed vertically in the hole, water would rise in the pipe to the level at which it stands within the margin of the lower saucer, its rise being caused by the same hydrostatic pressure that raised the water in the well at Southampton, coming from subterranean sheets of the fluid which exists in the fissured chalk-beds of the Hampshire basin, as they do also in the chalk under the basin of London."

Should these exceptional and assumed conditions occur in nature, the result would be substantially as indicated; but, as will be seen at a glance, the flow from a well sunk under such circumstances would be limited to the amount of water between the two saucers, and this will be limited to the quantity of rainfall. Since flowing wells and springs are seldom if ever thus limited, we infer that the case supposed does not occur. But whether it does or not is of no importance, since it in no sense satisfies the conditions of the "majestic column at Grenelle, and other cases where the flow is perpetual. We must, therefore, look for some other force to explain this class of phenomena. Professor Faraday followed Professor Buckland's lead six years later. M. Garnier, the celebrated French engineer, whose essay in 1822 upon this subject took the governmental prize, also takes this position; as does Dr. Halley. This theory we combat not merely from speculative motives, but in the interest of public health.

Various other theories have been advanced besides hydrostatic pressure. Aristotle and Seneca suggested the central heat of the earth. This theory has been more fully and scientifically stated by E, S. Chapin, in his work on gravity. But this is not the force that we seek. It is inadequate, as the following simple experiment shows: If a moderately flowing spring is surrounded by an air-tight inclosure which shall contract, and terminate in a tube, and this tube be allowed to have a discharge some distance below the surface of the water in the spring before its inclosure, it will be found that the water-flow from the spring has been greatly increased, though no change of temperature has occurred. Again, it has been suggested that the overflow of springs was due to capillary action; but this can hardly need a serious consideration in view of the amount and character of those overflows.

There are three classes of water to be taken into account in this discussion: 1. The surface waters mainly influenced by rainfalls; 2.