shall not touch its sides or bottom, although almost all the water will thus have been made to run over the sides, and only a spoonful may remain, yet the scales will continue balanced; and this without any regard to the weight of the body plunged into the water, and though you hold it entirely clear of the pitcher, so that it touches it in no place; for the effect will be the same if what you plunge in be scooped hollow, and made water-tight, so as to displace the greater part of the water. A bladder blown up, and tied fast, for example, and held down in the water, so as to leave only a spoonful surrounding it, will keep the scales balanced just as well as a block of lead the same size. Thus if E F (fig. 6.) be a balance with two scales E and F, you may put a jar of water A in one of the scales F, and balance it with weights in the other scale E. Then pour out

I

a

all the water but an inch or two at the bottom, so that it stands at B instead of A, as in the jar B; the weights in E will be much too heavy for it: now take a crooked piece of wood GHI, and place it so that the thick part I is plunged to near the bottom of the jar, and make the water rise from b to a, as high as it stood before in the full jar A; the scale F will again balance the weights in the scale E, although there is only the small quantity of water in it that surrounds the block. And this does not depend on the weight of the block I, which is entirely supported by the stand GK; for whether it be made of wood or lead or card, the water, if it stands as high round it, balances the same weight as before. An easy way of trying this is, by putting a tumbler full of water in one scale, and balancing it with weights in the other, then pouring out all but two or three tablespoonsfull; the scale with the weights will of course sink; but if you now put a smaller empty tumbler in, the

other, so as to make the water rise round it to the brim, still holding it when immersed, the balance will be restored; and the small tumbler will not make the scale weigh heavier if it be filled with lead-shot; nor will it make the scale lighter, if, instead of glass, the smaller tumbler is made of thin wood or card.

There is yet another way of illustrating the effects of this property of fluids. We have seen how the displacing of any portion of a fluid by a fixed solid, whatever be the weight of the solid, produces no difference in the weight of the fluid. provided it stands at the same height as before, and how, raising the height of the fluid by plunging a solid into it, increases its pressure, or apparent weight. If the fluid is raised by pressing or forcing it upwards, in however thin a column, provided the vessel be kept full, and closed in all directions, the pressure of the fluid will be increased, and the apparent weight of the vessel will be increased, although nothing whatever

AM. The plate being at the bottom CD, water is poured into the vessel, so that it rises nearly to A B, but does not rise in the tube. It is then balanced by a weight in the scale L. If the rod EK is drawn up so as to raise the plate, and force some of the water into the tube, the water will seem to weigh more than it did; and to restore the balance, more weight must actually be put into the scale L. If the vessel is three inches diameter, every inch that the water rises in the tube will require more than four ounces to be added to the weight, whatever be the bore of the tube; for the pressure of the water in all directions will be increased by the weight of a body of water, whose height is the height of the water in the tube, and whose base is the extent of the surface of the water pressing on the top A B of the vessel. Now the top being three inches diameter, its surface is about 7 square inches; and a portion of water one inch high, and 7 square inches broad, is 7 cubic inches of water, which weigh about four ounces. Thus, raising the rod a foot will add three pounds to the apparent weight of

the water.

This principle, from its extraordinary illustrations, is called the Hydrostatical paradox; paradox being a word from the Greek, and signifying something, which, though true, ap

pears when first considered to be untrue. When we are told that any quantity of water, however small, may be so employed as to balance any quantity of water, however great, we are at first startled by the apparent impossibility of the statement. But when we come to examine it more closely, we find it to be accurately true; for the small tube in the foregoing figures may be made ever so narrow, and to hold ever so little water, while the wide tube communicating with it may be made ever so large, and holding ever so much water; and the level at which the water stands in both tubes will be the same. So in the scales you may plunge as large a body as you please into the vessel of water, and leave as little water in the vessel as possible; still, if what you leave stands as high as the whole quantity stood, it will, by weight and pressure together, produce as much effect as the whole quantity of fluid.

Every thing, under these circumstances, depending upon the height and the surface, and very little upon the bulk of the fluid, we may easily perceive what mischief may be done by a very small quantity of water, if it happens to be applied or distributed, so as to stand high, in however thin a body or column, and to spread over a wide but confined and shallow space. Suppose that, in any building, a very

small quantity of water has settled, and is confined to the extent of a square yard on the ground near the foundation, and suppose it to fill up the whole vacant space or crevice of no more than half an inch deep, between the ground and some part of the masonry; if you take a tube, however slender, of twenty feet long, and thrust it down into the water, and then fill it with water from above, you apply a force or pressure of above five tons under a space of only a yard square of the building, and destroy it as easily as if you had mined it with gunpowder. This may be easily tried with a hogshead or butt of water, or any other liquid, by fixing a small strong pipe in the bung-hole, and pouring water through it; when the water rises in the pipe to a sufficient height (and this will be more or less according to the strength of the barrel), the barrel will burst, although but a very small quantity of water may have Deen poured into the pipe; for the pipe may be of an extremely small bore,

its width being wholly immaterial. One, twenty feet long, was found to burst a hogshead with great violence.

The same effect may be produced naturally by the rain falling into and filling some long narrow chink that may have been left in the walls of a building, or may be made by its decay in the course of time; and whether the chink be equally wide throughout, or vary in its size, and whether it be straight like a pipe, or crooked, makes no difference: provided it is water-tight, so as to get full of the rain, the pressure will always be in proportion to its perpendicular height, and not to its length if it winds. The same process in nature may produce the most extensive devastation; it may cause earthquakes, and split or heave up mountains. Suppose, in the bowels of some mountain, (fig. 8.) there should be an empty space of ten yards square, and only an inch deep on an average, in which a thin layer of water had lodged so as to fill it entirely; and suppose,

that, in the course of time, a small crack of no more than an inch in diameter should be worn from above, 200 feet down to the layer of water; if the rain were to fill this crack, the mountain would be shaken, perhaps rent in pieces with the greatest violence, being blown up with a force equal to the pressure of above 5022 tons of water, though only about 2 tons altogether had been actually applied. The same thing would happen if any one on the spot where there is such a layer of water below ground should bore down in sinking a well, or seek ing for a spring, and then fill the tube with water; it is impossible to fix the limits to the convulsion which

might ensue. This prodigious power however may be employed safely, and even beneficially. In the operations of nature, it is probably an important agent, though it has not been sufficiently attended to by philosophers in their attempts to explain natural appearances; and it is capable of being applied advantageously in the operations of art. It may plainly be used with great effect in mining. On a smaller scale, and as a power in machinery, it may certainly be employed far more extensively than it has hitherto been. A tube of a yard long, acting on a cavity of a yard square, will give a pressure equal to the weight of of a ton avoirdupois, if used

with water, but quicksilver may be employed instead of water, and as it is between thirteen and fourteen times heavier, we shall have a power of ten tons, by the use of a tube and a few pounds of mercury; and in like manner the power of a ton weight may be obtained within the space of a square foot in breadth, by a tube a little less than three feet long, and of the bore of a common goose quill.

The instrument, or rather plaything, called the Hydrostatic Bellows, is constructed upon the same principle. It consists of two boards attached to one another by leather, going all round them, and making the space within water-tight; there is no valve as in the air-bellows, but instead of it, a hole is bored in the upper board, and a pipe inserted, through which water is poured so as to fill the space between the boards. If the boards be a foot and a half long, and sixteen inches broad, and you load the upper one with three hundred weight, a quarter of a pound of water poured through the tube, and rising only three feet in it, will raise the whole weight as high as the leather allows. this way it will raise two stout men; and if, instead of pouring water into the pipe, the two men stand upon the upper board, and one of them blows into the pipe, the pressure thus made upon the water being conveyed in every direction, will produce the same effect, and raise them both. The smaller the bore of the pipe, the easier will they be raised, and by stopping it with the finger immediately after blowing, so as to keep in the air, they may keep themselves raised up. So when water is poured in, if the pipe be ever so small, and contain ever so little water, provided it be long enough, the weight will be raised by it.

In

A more striking as well as accurate manner of exhibiting this experiment was contrived by Ferguson, a man of great genius, who from the humble condition of a shepherd's boy raised himself to rank with the most useful philosophers of his age, and composed a work upon the different branches of Natural Philosophy that still holds a high place among the books which treat of those sciences, although he never had any further education from teachers than three months' reading and writing. A tube A B* (fig. 9.) is fixed upright in the

As the tube A B could not have sufficient length

and inserted in the neck of a bladder L, upon which is laid a board OP, and upon the board different weight mn, through a hole in each of which the pin IK, fixed in the board, passes; an arm G 1, passes from the box to steady the pin; water is then poured through AB till it fills the bladder, and the bladder is stretched, and raises the board, as soon as the water rises in the tube, although the weights may be above sixteen pounds, and the water in the tube not a quarter of an ounce.

The uses to which this power may be applied are of great variety and extent; and this branch of art appears however, been a most valuable and as yet to be in its infancy. There has, ingenious application of it by the late Mr. Bramah, in what is called the Hydrostatic Press, by which a prodigious force is obtained, strictly upon this principle, with the greatest ease, and within a very small compass; so that a man shall, with a machine the size of a on the table, cut through a thick bar common teapot, standing before him of iron as easily as he could clip a piece of pasteboard with a pair of

shears. The machine as most com

monly used is thus constructed. E F (fig. 10.) is a solid mass of wood or masonry, rendered steady by its weight, or by being fixed in the ground. B moveable up and down in grooves of the represents a strong horizontal board, two uprights; and any substance to be pressed or broken, is placed in the which B rests, moves up and down in space above it. The piston A, on the hollow cylinder L, and fits the neck N, so as to be water-tight. From the cylinder runs a tube, of much less bore

without encroaching too much upon the page, it is represented as if a part of it betwixt the extremites had been removed.

than the cylinder L, having at the part I a valve opening towards the cylinder; and D is the handle of a forcing pump CH, by means of whose piston water can be forced under the piston A. K represents another valve which, relieved from the pressure of the adjacent screw, allows the water to flow back again through the pipe M into the reservoir G, when the solid A is required to descend. The pressure upon the bottom of the piston at L, will be to the pressure upon the water in H, by means of the piston rod C, as the size of the under surface of A, to the size of the surface H, or as the section of that part of the cylinder occupied by the respective pistons. It is therefore as if we had to compare the pressure of water of the same depth, but on different surfaces; and this is in proportion to the surfaces. If the piston H is half an inch diameter, and the cylinder A one foot, the pressure of the water on the bottom of the cylinder will be to the pressure of the smaller piston on the water at H, as a square foot to a quarter of a square inch (the areas of circles being as the squares of their diameters), that is, as 144 square inches to a quarter of a square inch, or as 576 to 1; and therefore if the pressure of a ton weight be given by

means of the lever D, the cylinder A will be moved upwards, and be forced or pressed against whatever is placed in the space above it, with the weight of 576 tons. It is evident that this power may be increased without any other bounds than the strength of the materials, either by machinery, which will increase the force upon the water in the pump CH; or by increasing the disproportion between the diameters of the two pistons, or by both. Thus, if a pressure of two tons be given by a pump of only a quarter of an inch, and the cylinder be a yard in diameter, the pressure upwards will be equal to the weight of 41472 tons; and this prodigious effect will be produced by the agency of less than a pound of water. Such a force is much too great for the strength of any materials which we can employ. But within the space of nine or ten inches square and a foot high, a force of 5 or 600 tons may easily be brought to bear upon any substance which it is wished to press, to tear up, to cut in pieces, or to pull asunder.

Upon the tendency of all the parts of fluids to dispose themselves in a plain or level surface, depends the making of levelling instruments, or instruments for ascertaining whether any surface is level, or any line horizontal; for finding