This cannot be accounted for by saying that the wood or the glass was equal to the water displaced, for if we use lead, which is much heavier, or cork, and even card, which are much lighter, we shall meet with no difference. This property belongs to the water; and as the only constant fact was the same height of the fluid, to it must the explanation be referred; and we thus arrive at a first principle, a law in hydrostatics--that the pressure, or weight considered as a power, of any fluid, is not in proportion to its quantity, but to its depth. Aware of this principle, if we wish to use water as a power, we can economize it wonderfully, exerting a great pressure with a small quantity. If we take a small wooden box, water-tight, bore a hole in it, and fill it with water, adapt a long narrow tube to the hole, and fill it up with water, the box will now be burst, and that by the very small quantity contained in the tube. This tube may be a yard long, and very narrow in diameter, not holding more than two ounces of fluid, yet the pressure, being always in proportion to its depth, is the same as if it had been as broad as the box. This pressure amounts to nearly one pound on the square inch for every two feet of water. In the deepest parts of the ocean the pressure must be exceedingly great, so much so that it is probable they are uninhabitable, the pressure being too great for the existence of fishes. This pressure, together with the total absence of light at great depths, renders the existence of vegetable life also a doubtful matter. There is a certain depth beyond which divers cannot go, owing to the pressure of water on the surface of their chests being greater than the resistance of air inside, respiration being thereby impeded. A pipe a yard long, and acting on a yard square of fluid, will give a pressure equal to the weight of fifteen cwt. if we use water. Should we use quicksilver, the power of a ton weight may be obtained within the space of a square foot in breadth, by a tube somewhat less than three feet long, and not larger than a common goose quill--the pressure per square inch in these cases depending on the height of the column of fluid.