placed in a small cistern, previously filled quite full of water, when a quantity of it will flow over, and on again removing the floating body a vacuity of water will be found, which will be exactly reinstated by the quantity in the scale, being the weight of the floating body.

The balance which has been stated to take place among the columns of water may be pleasingly illustrated by the simple expedient of tying a bladder in a flaccid way over the end of a large patent lamp glass, or other cylinder which is open at both ends; when, upon filling the same to a little above the bladder, it will be borne down by the weight of the water, and will continue in the same situation even when the apparatus is immersed in water, until such immersion causes the water, both within and without the glass, to stand at the same level; and, whenever this is the case, a balance occurs between the pressures of the internal and external water, and the bladder will become quite flaccid, thus indicating that it is under no pressure either from above or below; on pressing the glass a little deeper into the water the external columns will become the longest, and consequently the most powerful, and the bladder will therefore in this case he as forcibly protruded upwards into the glass as it was at first pressed downwards.

The ancient method of supplying towns with water was by means of aqueducts, or bridges built over the valleys, and supporting either pipes or an open conduit or channel. These stupendous and costly erections, the remains of which still adorn the ruins of some ancient cities, and which exist in a more perfect state in the neighbourhoods of Paris and Lisbon, could not have been constructed for want of a knowledge of fluids rising to their common level, but probably from the practical difficulty of uniting a long range of pipes, in such a manner as to remain perfectly water-tight against the pressure of a heavy column of water, a circumstance which is by no means easy, even in our present state of improvement, and with all the advantage of cast iron and the most durable materials, instead of stone or earthenware, which appears to have been chiefly resorted to for pipes in the formation of the older water-works.

The New River water-works, which are of such vast importance to the comfort and health of the great metropolis of England, are in themselves a species of aqueduct, and unite all the varieties in the construction of water-works. The spring that supplies them rises at Ware, in Hertfordshire, and its waters are conducted in an artificial channel or cut, formed for their conveyance alone, which is sometimes raised by arches and embankments very considerably above the natural surface of the ground, and at others sinks deeply into it, for upwards of thirty-eight miles. At length it ends in the open basin or reservoir, called the New River Head, at Islington, which is sufficiently high to supply the lower parts of the town by its natural descent into the pipes. To accomplish the rest, a powerful steam-engine is placed near this reservoir, for the purpose of working pumps which force a part of the water into still more elevated reservoirs on Pentonville Hil, and in the Hampstead read, and what these

cannot command is effected by an air vessel attached to the pumps of the steam engine, so that the greater part of London is supplied, without the expense of any other power than the water's natural gravitation, and the remainder by the well appropriated power of a steam engine.

There is a very singular paradoxical experiment illustrative of this part of our subject. It is this, that any quantity of water, or any other fluid, however small, may be made to balance and support any quantity, or any weight, how great soever. Thus, the water in a pipe, or canal, open at both ends, always rises to the same height at both ends, whether those ends be wide or narrow, equal or unequal. And since the pressure of fluids is directly as their perpendicular heights, without any regard to their quantities, it follows, that whatever the figure or size of the vessels may be, provided their heights be equal, and the areas of their bottoms equal, the pressures of equal heights of water are equal upon the bottoms of those vessels, even though the one should contain 1000, or 10,000 times as much as the other. Mr. Ferguson has illustrated this matter by the following apparatus:-Let two vessels, plate 1, figs. 3 and 4, such as C and O, be of equal heights, but very unequal capacity; let each vessel be open at both ends, and their bottoms E and F of equal widths; let the brass bottoms be exactly fitted to each vessel, not so as to go into them, but for each vessel to rest upon respectively; and let a piece of wet leather be put between each vessel and its brass bottom, for the sake of keeping them close. Join each bottom to its vessel by a hinge, A F, so that it may open like the lid of a box; and let each bottom be kept up to its vessel by equal weights, BI, hung to lines which pass over the pulley as at I, the blocks being fixed to the sides of the vessel, and the lines tied to hooks at DB, fixed in the brass bottoms opposite to the hinges. Things being thus prepared, hold one vessel upright in the hand over a basin on a table, and cause water to be poured slowly into it, till the pressure of the water bears down its bottom at the side, and raises the weight, and then part of the water will run out beneath. Mark the height at which the surface of the water stood in, the vessel when the bottom began to give way; and then, holding up the other vessel in the same manner, cause water to be poured into it, and it will be seen that, when the water rises in this vessel just as high as it did in the former, its bottom will also give way at the same height, and it will lose part of the water.

The cause of this apparently surprising phenomenon is, that, since all the parts of a fluid at equal depths below the surface are equally pressed in all directions, the water immediately below the fixed part will be pressed as much upward, against its lower surface within the vessel, by the action of the column in the centre, as it would be by a column of the same height, and of any diameter whatever; and therefore, since action and re-action are equal, and contrary to each other, the water immediately below the surface, B, will be pressed as much downwards by it as if it were immediately touched,

and pressed by a column of the whole height, and of the diameter A B; and therefore the water in the cavity beneath will be pressed as much downwards upon its bottom, F, as the bottom of the other vessel is pressed by all the water above it.

When a machine is constructed expressly for the purpose of showing, in the most striking manner, that the pressure of fluids is as their perpend.cular heights, and that a quantity, however small, may be made to support a weight, or another quantity, however large, it may be most advantageously made in the form of what is called the hydrostatical bellows. This apparatus may now be examined. It consists of two circular boards I, plate I. fig. 5, about sixteen inches in diameter; these boards are connected by means of a strong leather, which entirely surrounds them, and permits them to open and close like a pair of common bellows, with this difference, that they open equally all round, and therefore the boards always remain parallel to one another. The leather, at its junctures, is well secured, and the whole machine is watertight. In the upper board is fixed a pipe, AC, communicating with the interior, and reaching above to a considerable height, suppose five feet. Through this pipe let some water be poured into the bellows, and the upper board will be observed to rise a little; place a weight, B, upon it, pour in more water, and it will rise, though we increase the weight very considerably. Indeed, we may add water till the leathers are at their utmost extension; the water will then fill in the tube, and the upper board cannot be depressed, nor the water forced out of the small tube, until the pressure upon it is more than that of a column of water whose diameter is equal to that of the interior of the bellows, and its height equal to that in the tube; by increasing, therefore, the length of the tube, a most enormous weight might be raised by the pressure of a few ounces of water.

To illustrate this singular experiment, we may suppose a hole to be made in any part of the upper board, and another tube to be inserted there; the water would certainly rise to the same level in them both; and, supposing the board to be filled with tubes, the water would obtain the same level in them all, because a series of pipes would, in fact, form a solid cylinder of water. If we suppose the hole to be of the same diameter as the interior of the tube, and fitted with a piston, then, if the tube contained two ounces of water, the piston would sustain a weight of two ounces without being depressed. If the area of the whole were twice that of the bore of the tube, two ounces in the tube would sustain four ounces on the piston. In this manner, every part equal to the bore of the tube is pressed upwards with a force equal to the weight of fluid in the tube. Hence, if the proportion subsisting between the area of the tube and that of the bellows be multiplied by the weight of water in the tube, the product will express the force with which the boards are separated.

In lieu of the bellows part of the apparatus, the leather of which would be incapable of re

sisting any very considerable pressure, Mr. Bramah suggested the use of a very strong metal cylinder, in which a piston was so packed as to move water-tight; and, as a substitute for the high column of water, he employed a small forcing pump, to which any power can be applied; and thus the pressing column becomes indefinitely long, although the whole apparatus is of itself comparatively small.

In plate I., fig. 6, we have a section of one of these presses, in which b is the piston of the large cylinder, formed of a solid piece of metal turned truly cylindrical, and carrying the lower board v of the press upon it: r is the piston of the small forcing pump, being also a cylinder of solid metal, moved up and down by a handle or lever. The whole lower part of the press is sometimes made to stand in a case, S a, containug more than a sufficient quantity of water, as at C, to fill both the cylinders; and the suction pipe of the forcing-pump dipping into this water will be constantly supplied. Whenever, therefore, the handle is moved upwards, the water will rise through the conical metal valve, opening upwards into the bottom of the pump t; and, when the handle is depressed, that water will be forced through another similar valve g, opening in an opposite direction in the pipe of a communication between the pump and the great cylinder d, which will now receive the water by which the piston rod b will be elevated at each stroke of the pump t. Another small conical valve, f, is applied by means of a screw to an orifice in the lower part of the large cylinder, the use of which is to release the pressure whenever it may be necessary; for, on opening this valve, any water which was previously contained in the large cylinder d will run off into the reservoir by the passage c, and the piston b will descend; so that the same water may be used over and over again. The power of such a machine is enormously great; for supposing the hand to be applied at the end of the handle with a force of only ten pounds, and that this handle, or lever, be so constructed as to multiply that force but five times, then the force with which the piston r descends will be equal to fifty pounds: let us next suppose that the magnitude of the piston b is such, that the area of its horizontal section shall contain a similar area of the smaller piston r fifty times; then fifty times multiplied by fifty gives 2500 pounds, for the force with which the piston b and the presser v will rise. A man can, however, exert ten times this force for a short time, and could therefore raise 25,000 pounds; and would do more if greater disproportion existed between the two pistons b and r, and the lever or handle of the pump were made more favorable to the exertion of his strength.

Mr. Hawkins has contrived an hydrostatic weighing machine, which may be easily understood by reference to plate 1. fig. 7: a is a cylinder made of tin, and japanned, which is partly filled with water; b is another cylinder, rather less in diameter, resting upon and floating in the water contained in the external vessel A graduated scale and glass tube, c, are seen to ru parallel to the vessel a, with which they are con

nected at bottom; so that the water always stands at the same height in both. The central cylinder is furnished with a dish or scale for holding the goods to be weighed, the pressure of which causes it to sink. The water being thus displaced in the larger vessel is raised in the small tube in an equal proportion, and the exact weight of the goods will be indicated by the scale attached to the tube. This ingenious contrivance is well adapted for small weights; and, if mercury be substituted for the water recommended by Mr. Hawkins, the range of the instrument may be extended, without its accuracy being affected by its evaporation.

If a hole be made in the side of a vessel, the water will spout forth in a curvilinear path because fluids press equally in all directions.

The velocity with which water spouts out, at a hole in the side or bottom of a vessel, is as the square root of the depth or distance of the hole below the surface of the water: for, in order to make double the quantity of a fluid run through one hole as through another of the same size, it will require four times the pressure of the other, and therefore the aperture must be four times the depth of the other below the surface of the water; and, for the same reason, three times the quantity, running in an equal time through the same sort of hole, must run with three times the velocity, which will require nine times the pressure, and consequently the hole must be nine times as deep below the surface of the fluid, and so on.

To prove this by experiment, let two pipes of equal sized bores be fixed into the side of a vessel, one pipe being four times as deep below the surface of the water in the vessel as the other is: and, whilst the pipes run, let water be poured constantly into the vessel, so as to keep it always full. Then if a cup that holds a pint be so placed as to receive the water that spouts from the upper pipe, and at the same moment a cup that holds a quart be placed to receive the water from the lower pipe, both cups will be filled at the same time by their respective pipes.

The horizontal distance to which a fluid will spout from a horizontal pipe in any part of the side of an upright vessel, below the surface of the fluid, is equal to twice the length of a perpendicular to the side of the vessel, drawn from the mouth of the pipe to a semicircle described upon the altitude of the fluid and therefore the spout will be to the greatest distance possible from a pipe whose mouth is at the centre of the semicircle; because a perpendicular to its diameter (supposed parallel to the side of the vessel), drawn from that point, is the longest that can possibly be drawn from any part of the diameter to the circumference of the semicircle.

Thus if the vessel AB, plate I. fig. 8, be full of water, the horizontal pipe D be in the middle of its side, and the semi-circle NEC be described upon D, as a centre, with the radius, or semidiameter DC, or DN, the perpendicular DE to the diameter CDN is the longest that can be drawn from any part of the diameter to the circumference: and, if the vessel be kept full, the jet will spout from the pipe D to the horizontal distance M M, which is double the length of the

perpendicular DE. If two other pipes, as F and G, be fixed into the side of the vessel, at equal distances above and below the pipe D, the perpendiculars FH and G I, from these pipes to the semi-circle, will be equal; and the jets spouting from them will each go to the hori zontal distance N K, which is double the length of either of the perpendiculars FH or GI.

The reader will easily perceive that the curve described by the spouting fluid, in all the different situations, will be that of a parabola; being acted upon by the combined forces of the lateral pressure of the fluid in the vessel, and the force of gravity.

When a solid body is plunged in any liquid, it must displace a quantity of that liquid exactly equal to its own bulk. Hence, by measuring the bulk of the liquid so displaced, we can ascertain precisely the bulk of the body; for the liquid can be put into any shape, as that of cubical feet or inches, by being poured into a vessel of that shape, divided into equal parts, or the vessel in which the body is plunged may be of that shape, and so divided. If the width of the vessel is four inches by three, or twelve square inches, and divided on the upright side into twelfths of an inch, when a body of any irregular shape, as a bit of rough gold or silver, is plunged in it, every division that the water rises will show that one-twelfth of twelve cubic inches, or one cubic inch of water, has been displaced; so that, if it rises two divisions, the body contains exactly two solid inches of metal. And this is by far the easiest way of measuring the solid contents of irregular bodies.

When a body is so plunged it will remain in whatever part of the fluid it is put in, if it be of the same weight with the fluid; that is, if the bulk of the body weighs as much as the same bulk of the fluid; for in this case it will be the same thing as if the fluid were not displaced, and as an equal quantity of the fluid would have remained at rest there, being equally pressed on all sides, so will the solid body: it will be pressed from below with the same weight of fluid as from above. But if the body be heavier than the fluid, bulk for bulk, this balance will be destroyed, and the weight of the fluid pressing from above will be greater than that pressing from below, by the weight of the solid body, which will therefore sink to the bottom. So, if it be lighter than an equal bulk of the fluid, it will rise through the fluid to the surface. But if a solid heavier than the fluid be plunged to a depth as many times greater than its thickness, as the solid is heavier than the fluid, and there protected by any means from the pressure of the fluid above, it will float notwithstanding its weight, because the pressure from below, being in proportion to the depth, will counter-balance the weight of the body, and there will be no pressure from above, except the weight of the body. Thus, lead is somewhat above eleven times heavier than water. If a cube of lead be placed so as to press closely against the bottom of a wooden pipe one foot square, closed at the top, and plunged twelve feet deep, and held upright, it will there swim; the water pressing it upwards with a force greatcr than its weight, and there being no pres

sure fron. the water downwards. So if a body lighter than water, as cork, be placed at the bottom of a vessel, and so smoothly cut that no water gets between its lower surface and the surface of the bottom, it will not rise but remain fixed there, because it is pressed downwards by the water from above, and there is no pressure from below to counter-balance that from above. It follows from these principles, that if any body be weighed in the air, and then weighed in any liquid, it will seem to lose as much as an equal bulk of the liquid weighs. Not that the body really loses its weight, but that it is pressed upwards by a force equal to the weight of the liquid, the place of which it fills. Thus, if a piece of lead weigh an ounce before being plunged in water, that is, require an ounce weight on the opposite scale to balance it; if you hang it by a thread from its own scale, and let it be plunged so that the water in a full jar covers it, a quantity of water equal to the bulk of the lead will run over the sides of the jar, and a number of grains equal to the weight of this quantity of water must be taken out of the opposite scale to restore the balance: for the lead is now pressed downwards in the water with a force not equal to its own weight, but to the difference between its own weight and that of an equal bulk of the water. And in this manner we can determine the relative weights of all bodies, or the proportion which they bear to each other in weight; which is called their specific gravity; that is, their weight in kind, and some times their relative gravity, that is, their weight compared with the weight of other bodies. By weighing a known bulk, as a cubic foot or a cubic inch of any two substances, we can find their specific gravity; or their gravity as compared with each other: if, for instance, we found a cubic inch of iron weighed 1948 grains, and a cubic inch of lead 2858, we should say, that the specific gravities of the two substances were in the proportion of 3 to 4 nearly; and so we might find the specific gravity of a solid substance, as compared with that of a liquid, by weighing an equal bulk of each. But this operation is extremely difficult, because it requires the substances compared to be formed accurately into the same shape and size; and, when we are not allowed to change their figure, the comparison cannot be made at all. Thus we could not ascertain the specific gravity of precious stones, crystals, metallic ores, or animal and vegetable substances, without in effect destroying them. But the hydrostatic balance, upon the principles now explained, affords a perfectly easy and most accurate method of comparing all substances solid and fluid. We have only to weigh any substance first in air, and then in water; the difference of the weights is the weight of a bulk of water equal to the bulk of the substance; and by comparing any other substance with water, in like manner, we ascertain its specific gravity, as compared with that of

the first substance.

Let us now take an example of the use of the hydrostatic balance. If a guinea suspended in air be counterbalanced by 129 grains in the opposite scale, and upon being immersed in water re

quires 74 grains to be put in the scale over it, in order to restore the equilibrium; we thus find that a quantity of water, of equal bulk with the guinea weighs 7 grains, or 7-25; by which divide 129, the weight of the guinea in air, and the quotient, or 17-793, shows that the guinea is so many times heavier than its bulk of water. Whence if any piece of gold be tried, by weighing it first in air, then in water, and if upon dividing the weight in air by the loss in water, the quotient is 17.793, the gold is good; if the quotient is 18, or between 18 and 19, the gold is very fine; but, if it be less than 17, the gold is too much alloyed with other metal. If silver be tried in this manner, and found to be eleven times heavier than water, it is very fine: if it be 10 times heavier, it is standard: but, if it be any less weight compared with water, it is mixed with some lighter metal, such as tin.

When substances are lighter than water, a different mode of treatment to that which has been described must be adopted for obtaining their specific gravities; for now some force is necessary for producing their submersion :-To effect this a small pulley moving with little friction may be attached to the bottom of the water jar, or to a weight sufficiently heavy to cause it to remain steadily there; and the hair attached to the substance, must in this case pass downwards under the pulley, and rise again so that its opposite end may fix to the hook of one of the scale pans. The substance is first to be weighed in the scale in the ordinary manner, and afterwards placed in the jar; water must then be added, until the substance, by floating, draws the scale beam into an horizontal position; after which weights must be placed in the opposite scale until the substance is drawn under the water.

Mr. Ritchie's hydrostatic balance is cheap and delicate; it is constructed as follows:-Let a slender beam of wood be procured, about eighteen or twenty-four inches long, and tapering a little from the middle to each end. Let a fulcrum of tempered steel, resembling the blade of a pen-knife, be made to pass through the middle of the beam, a little above the centre of gravity. Similar steel blades are also made to pass through the ends of the beam for suspending the scales. The fulcrum rests on two small portions of thermometer tubes, fixed horizontally on the upright support E F, fig. 9, plate I. The support has a slit passing along the middle, to allow the needle at EF to play between the sides. A small scale made of card, and divided into any number of equal parts, is placed at F, for the purpose of ascertaining the point at which the needle remains stationary. This balance possesses extreme delicacy. It may even be made more sensible than that belonging to the Royal Society of London.

To ascertain the weight of any body, place it in one of the scales, and bring the needle to any point by means of small shot placed in the other scale; observe the point opposite to which the needle rests, or the middle between its extreme point of oscillation; remove the body, and put into the scale as many known weights as will bring the needle to the same division as