The minister of some pure temple, where
No human errors mingle with the work.
ON THE POWER OF FLUIDS.
That weight is a property of liquids, has been acknowledged by the earliest observers; but the amount of that weight, its mode of acting, and application to practice, have been left for recent times to discover. A pint of water weighs somewhat more than a pound avoirdupois; and one unacquainted with the facts in hydrostatics might deem it of little consequence what shape the vessel that contained it might be, or what the disposition and length of the column of water—for, after all, what is it but a pound of water? No idea can be more erroneous. Under most circumstances, it is not so much the quantity of the fluid as the manner in which its particles are disposed, that determines its weight; and what may appear still more extraordinary, a small quantity of fluid may be made to balance, that is, to be of the same apparent weight as, a very large quantity. This may be proved by taking a pair of scales, putting a tumbler full of water into one dish, and balancing it by weights in the other, then inverting a smaller glass and immersing it in the tumbler, having the glass perfectly supported in the hand to prevent it touching the sides or bottom; a portion of the water will now flow over the sides of the tumbler—say one-half—yet the scales are still balanced; one-half of the water is of the same weight apparently as the whole. A piece of wood may be used instead of the glass with the same result, and it may be of a size nearly to fill the cavity of the tumbler; yet if the remaining water, which may amount to no more than a couple of spoonfuls, rise to the same level as it did when full, it will exactly balance the weights. 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.
We can now understand what extensive and sometimes irremediable injury may arise from the collection of a small but lofty column of water, opening into a wide but confined space below. This sometimes occurs when water gets into a narrow chink between buildings, and, finding its way down, opens finally into some cavity under the floor. The pressure exerted here is immense, and there are few bodies able to resist it. It is owing to this that the pipes for conveying water are burst, on account of the pressure exerted on the insides of the pipes; and this occurs the more frequently, the higher the source from which they are filled. In practice, every vessel containing liquid should increase in strength in proportion to its depth. We have no doubt that a process similar to this takes place on the large scale in nature, which is capable of uprooting trees, rending rocks, producing earthquakes; for if we suppose that some collections of water on the surface of a hill have found their way down through crevices into a cavity in the body of the mountain which has no external opening, as long as this cavity remains unfilled no evil arises, but when it and the crevices also are completely filled, the pressure exercised here is so immense, that even the sides of the hill cannot withstand it. Perhaps this occurrence has not been sufficiently noticed in explaining natural phenomena. It is usual to consider earthquakes and volcanoes as solely the result of chemical action, excluding entirely physical agency.
The pressure of water may be rendered visible by blowing through a tube under water into a tall glass jar. The bubble of air, small at the bottom, as it rises, gradually enlarges from the diminution of the pressure.
The hydrostatic bellows, formed upon this principle, consists of nothing more than a water-tight bellows, with a long pipe fixed into the valve aperture. If this pipe be three feet long, and hold a quarter of a pint of fluid, it will exert a pressure sufficient to raise three cwt. laid upon a bellows, the area of the upper side of which is equal to about a square foot and a half. Many are the uses to which this principle might be applied in the several arts.
Bramah’s Press is almost the only machine which has been extensively used. By its means solid bars of iron can be cut through with ease. Hay and cotton have been compressed by its means into a very small compass. In the East Indies, where water-power is used, bales of cotton are compressed into one-half the size of those from the West Indies. By its means power may be multiplied, or rather concentrated, a thousand-fold. As commonly made, a man working it may, by using the same force that would raise half a cwt., apply a force amounting to twenty tons to the work in hand; and by varying the proportions of the machine, pressure might be brought to bear upon any body which would be perfectly irresistible.