129. =Great Effects from Small Quantities of a Fluid.=--You are now prepared to understand the explanation of some very striking phenomena in the pressure of liquids. If you take a perfectly tight cask, and, filling it with water, screw into its top a long tube, by pouring water into the tube you can burst the cask. To understand this you must bear in mind two facts--that the fluid in the cask is not compressible, and that its particles move freely among each other. Any pressure, therefore, exerted upon it is felt through the whole of it equally. "If the tube," says Dr. Arnot, "have an area of a fortieth of an inch, and contain when filled half a pound of water, this produces a pressure of half a pound upon every fortieth of an inch all over the interior of the cask; which is more than a common cask can bear." Suppose a small reservoir of water exists in the side of a mountain wholly closed up, and that water from a height above finds its way to it by a crevice, it may by its pressure even burst open the side of the mountain. And it matters not how large or small the crevice may be, for pressure in a liquid is only as the height. If the reservoir be ten yards square and an inch deep, and the fissure leading to it be but an inch in diameter and two hundred feet in height, it is calculated that the pressure of the water in the fissure would be equal in force to the weight of 5000 tons. [Illustration: Fig. 83.] 130. =Explanation.=--The manner in which these effects are produced may be made clear by Fig. 83. Let A be a close vessel filled with water, and let a tube, _b_, be made fast in it, with a movable plug or piston at _c_. If the surface of the water be pressed upon by this piston with the force of a pound, as the water is incompressible and its particles are freely movable among each other, the pressure will be extended equally through all the water, and every portion of the vessel of equal extent with the tube's opening at _c_ will be pressed upon with the force of a pound. If another tube, _d_, of the same size were inserted with a piston, _i_, the force of a pound applied to the piston _c_ would push upward the piston _i_ with the same force. And if there were several pistons of the same size, by pushing upon one with the force of a pound they would all be pressed upward with exactly this force. Farther, if _e_ be a tube five times as large as _b_, its piston, _n_, will be forced upward with a pressure of five pounds by the downward pressure of a pound upon _c_. Suppose now that a pound of water were substituted for the piston _c_, the other pistons would be pressed upward as before. And if all the pistons be removed, the pound of water in _b_ will press the water up the tube _d_ with the force of a pound, and up the tube _e_ with the force of five pounds. [Illustration: Fig. 84.]