119. =Aqueducts.=--The ancients built aqueducts of stone at immense expense, in some cases spanning valleys at great heights, to supply their cities with water. At the present day the same object is effected at comparatively small expense with iron pipes laid under ground. No matter how much lower than the reservoir a valley crossed by the pipes may be, the water flowing through them will rise any where in their branches to the same height as it stands in the reservoir. It is supposed by some that the ancients were not aware of this fact; but by others that they were aware of it, and built their immense aqueducts because they had no material for constructing large pipes. [Illustration: Fig. 74.] 120. =Springs and Artesian Wells.=--The principles which I have developed in the previous paragraphs will explain the phenomena of springs, common wells, and Artesian wells. The crust of the earth is largely made up of layers of different materials, as clay, sand, gravel, chalk, etc. When these were formed they were undoubtedly horizontal, but they have been thrown up by convulsions of nature in such a way that they present every variety of arrangement. As some of these layers are much more pervious to water than others, the rain which falls and sinks into the ground often makes its way through one layer lying between two others which are impervious to water, and so may make its appearance at a great distance from the place of its entrance, and at a very different height. How this explains the phenomena of springs, common wells, and Artesian wells may be made clear by Fig. 74. A A and B B B are designed to represent porous layers of earth lying between other layers which are impervious to water. The water in A A will flow out at C, making what is commonly called a spring. If we dig a well at F, going down to the porous layer, B B B, the water will rise to G, because this is on a level with the surface of the ground, H, where the supply of water enters. From this point it may be raised by a pump. If the well be dug at D, the water will rise not only to the surface but to E, because this is on a level with H. Water is sometimes obtained under such circumstances from very great depths. In this case the porous stratum containing the water is reached by boring, and then we have what is termed an Artesian well. The name comes from Artois, in France, where this operation was first executed. There is a celebrated well of this sort in Paris over 1800 feet deep, and the water rises 112 feet above the surface. More than 600 gallons are discharged every minute. "London," says Dr. Arnot, "stands in a hollow, of which the first or innermost layer is a basin of clay, placed over chalk, and on boring through the clay (sometimes of 300 feet in thickness) the water issues, and in many places rises considerably above the surface of the ground, showing that there is a higher source or level somewhere--probably among the Surry hills or those north of London."