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.
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."
121. Pressure of Liquids in Proportion to Depth.—The pressure of a fluid is in exact proportion to its depth. For, as the particles are all under the influence of gravity, the upper layer of them must be supported by the second, and these two layers together by the third, and every layer must bear the weight of all the layers above it. The increase of pressure at great depths produces the most striking effects. Thus if an empty corked bottle be let down very deep at sea, either the cork will be driven in or the bottle will be crushed in. A gentleman tried the following experiment: He made a pine-wood cork, so shaped that it projected over the mouth all around. He then covered this with pitch, and fastened over the whole several pieces of tarpaulin. The bottle, thus prepared, he let down to a great depth by attaching to it a weight. On raising it up he found that it contained about half a pint of water strongly impregnated with pitch, showing that the pressure of the water forced water through the several pieces of tarpaulin, the pitch, and the pores of the wooden cork. When a ship founders near land, the pieces of the wreck, as it breaks up, float to the shore; but when the accident happens in deep water, the great pressure forces water into the pores of the wood, and thus makes it so heavy that no part of the vessel will ever rise again. When a man dives very deep he suffers much from the pressure on his chest. If we watch a bubble of air rising in water it is small at first, but it grows larger as it approaches the surface, because it sustains less pressure than when it was deep in the water. The force with which a fluid is discharged from an opening in a vessel depends on the height of the fluid above the opening. The difference in this respect between a full barrel and one nearly empty is very obvious. Most fishes, probably, can not bear the pressure of great depths, and so are commonly found on the coast, or on banks, as they are called, in the midst of the ocean.
Fig. 75.
122. Sluice-Gates, Dams, etc.—The application of the above principles in the construction of sluice-gates, dams, etc., is a matter of great practical importance. Let us look at this. As pressure in a fluid is always in proportion to the height of the fluid above the point of pressure, the pressure upon any portion of the side of a vessel containing a fluid must be in proportion to its distance from the surface; or, in other words, it is the weight of a column of water extending from this portion to the surface. Let A B C D (Fig. 75) represent a section of a cubical vessel, that is, one in which each side is of the same size with the bottom. The pressure on the point a, in the line A B, is that of a column of particles, A a. But A a is equal to c b, and c b is equal to b a. Therefore b a may represent the pressure on a. In the same way it can be shown that e d represents the pressure on d, n m the pressure on m, C B that on B. Therefore the pressure on all the points in A B will be represented by lines filling up all the triangular space A B C, and this is half of A B C D, which represents the pressure on the line C B. It is clear, then, that as the pressure on a vertical line in the side is half that on a line at right angles to it in the bottom, the pressure on the whole side is half that on the whole bottom.
Fig. 76.