« ΠροηγούμενηΣυνέχεια »
is represented in plate I. fig. 1. HYDROSTATICS part of its surface is equidistant from the centre and IIYDRAULICS. The end I, of a cylinder S, of the earth, or in what is generally called a three inches wide and eighteen long, being level plane, although that apparent plane is, in made water-tight by a plate firmly soldered to fact, not a plane, but partakes of the convexity it: a cap, also water-tight, was screwed on the of the earth. And it is for the purpose of estab extremity. The rod a, faths of an inch in dia- lishing such an equilibrium that fluids always meter, and carrying a flexible ring C, was made run from a higher to a lower situation. to pass through a tight stuffing-box F. The For the purpose of explaining the manner in compression was effected in a cannon, the top which the surfaces of fluids become level, it may of which was capable of containing the piezo- be very fairly supposed that the particles of which meter. It was fixed vertically in the earth, the they are composed are placed one upon another touch-hole being plugged tight, and the muzzle so as to form what may be termed pillars or coabout eighteen inches above ground. A strong lumns of particles as represented in plate I. fig. cap was firmly screwed on at the mouth, and in 2, and supposing all the particles to be of the the centre of it a small forcing pump, with a same size and weight, then the six which are on piston gths of an inch in diameter, was tightly one side will be an exact balance to the six screwed, and a valve introduced to ascertain the which are on the other (both columns being supdegree of pressure, one pound of pressure on ported by the bottom of the vessel which conthat valve indicating an atmosphere. In per- tains them), and their two tops will be level; forming experiments with this apparatus, the but, if the two upper particles t and v are suppiezometer was introduced into the cannon, the posed to be taken away, a balance can no longer water being forced in till the cap showed signs exist, for now there will be six particles in one of leakage: the valve at the same time indi- column, while there are but four in the opposite cating a pressure of 100 atmospheres; when one to press against and resist them, the consethe piezometer was taken out of the cannon, the quence of which will be that the tallest column Hlexible ring, C, was eight inches upon the rod a, will descend, and the particle u will fall into the which proved that the rod had been forced that situation of 'w, while that marked x will, with length into the cylinder, and that the compres- its column, ascend into the situation v, and thus sion was about one per cent.; in order to pro- x and u will come to the same level, and a baduce this compression, three per cent. must be lance or equilibrium will be again restored. pumped into the gun; an effect arising from the Every vessel is supposed to be filled by an infiexpansion of the gun, or the entrance of the nite number of such columns, although two only water into the pores of the cast iron.
are represented in the figure to prevent confusion. On his voyage to England, Mr. Perkins re- The cause of bodies floating upon fluids, or peated this experiment frequently, and with the sinking in them, may be explained by the same same result; by sinking the piezometer with reasoning, for whenever a solid is immersed in a fifty-four pounds of lead, to the depth of 500 fluid, it displaces a quantity of water, and confathoms, which gives nearly a pressure of 100 sequently renders the columns of particles underatmospheres. Being satisfied that the above neath it shorter, and therefore lighter, than those piezometer would not show all the compression, which surround it. It will only then be to conhe made another, consisting of a small tube, ceive that the two particles t and v, in the last closed at the lower end, and water-tight; at the figure represent a body which is partly immersed upper end, the water entered through a small in the water, and is Aoating near its surface; aperture, closed by a delicate valve opening in- the columns under that body will be shurter than wards; it was then perfectly filled with water those which surround it, but the weight of the (the weight of which was accurately known), body becomes a counterpoise to the greater and subjected in a common hydraulic press to a length of the surrounding columus, and must in pressure of about 326 atmospheres. When every case be precisely equal to the quantity of taken out, and weighed, there was found an in- water which it displaces, otherwise it cannot crease of water, amounting to about 34 per float, for all bodies which are incapable of so cent. This water had been previously boiled becoming this counterpoise, or, in other words, and cooled down to 48°, and kept at that tem are so heavy that their small bulk will not perperature during the experiment.
mit them to displace as much water as is equal Fluids have weight, and gravitate towards the to their own weight, must inevitably sink ; conseearth according to their density in the same way quently all things which are lighter than their that solids do; but from the want of cohesion bulks of water will swim, and all that are heavier among their particles they are, however, incapable must sink, unless when they are placed in a boat of assuming and retaining any particular form or or hollow vessel, which by its bulk enables them to figure without support and assistance, and conse- displace more water than is equal to their weight, quently they always take the form of the vessel and then they will float. A ship therefore of which contains them, and they also exert a cer 500 tons burthen must displace 500 tons of tain force against the sides of that vessel from water from the bed or hollow which it makes their tendency to fall, which constitutes their itself up to its water-line, and in this way the lateral pressure ; for fluids not only press down- tonnage of vessels is estimated. wards with their whole weight, in obedience to The truth of this position is very satisfactorily gravitation, but they press sideways or laterally proved by putting the model of a ship or any in all directions at the same time and from the other body capable of floating into a scale, and same cause, and consequently no Auid can re- exactly balancing it with water in the other scale. main in a state of quiet equilibrium, unless every The toating body is then to he removed and
placed in a small cistern, previously filled quite cannot command is effected by an air vessel full of water, when a quantity of it will now attached to the pumps of the steam engine, so over, and on again removing the floating body a that the greater part of London is supplied, vacuity of water will be füund, which will be without the expense of any other power than exactly reir.stated by the quantity in the scale, the water's natural gravitation, and the remainder being the weight of the floating body.
by the well appropriated power of a steam The balance which has been stated to take engine. place among the columns of water may be plea-' There is a very singular paradoxical experisingly illustrated by the simple expedient of ment illustrative of this part of our subject. It tying a bladder in a flaccid way over the end of is this, that any quantity of water, or any other a large patent lamp glass, or other cylinder which fluid, however small, may be made to balance is open at both ends; when, upon filling the same and support any quantity, or any weight, how to a little above the l·ladder, it will be borne great soever. Thus, the water in a pipe, or canal, down by the weight of the water, and will con open at both ends, always rises to the same tinue in the same situation even when the appa- height at both ends, whether those ends be wide ratus is immersed in water, until such immersion or narrow, equal or unequal. And since the causes the water, both within and without the pressure of fluids is directly as their perpendiglass, to stand at the same level; and, whenever cular heights, without any regard to their quanthis is the case, a balance occurs between the tities, it follows, that whatever the figure or size pressures of the internal and external water, and of the vessels may be, provided their heights be the bladder will become quite faccid, thus indi- equal, and the areas of their bottoms equal, the cating that it is under no pressure either from pressures of equal heights of water are equal ahove or below; on pressing the glass a little upon the bottoms of those vessels, even though deeper into the water the external columns will the one should contain 1000, or 10,000 times as become the longest, and consequently the most much as the other. Mr. Ferguson has illustrated powerful, anu the bladder will therefore in this this matter by the following apparatus :-Let ease he as forcibly protruded upwards into the two vessels, plate 1, figs. 3 and 4, such as C and glass as it was at first pressed downwards, O, be of equal heights, but very unequal capa
The ancient method of supplying towns with city; let each vessel be open at both ends, and water was by means of aqueducts, or bridges their bottoms E and F of equal widths; let the built over the valleys, and supporting either pipes brass bottoms be exactly fitted to each vessel, or an open conduit or channel. These stupen- not so as to go into them, but for each vessel to dous and costly erections, the remains of which rest upon respectively; and let a piece of wet still adorn the ruins of some ancient cities, and leather be put between each vessel and its brass which exist in a more perfect state in the neigh- bottom, for the sake of keeping them close. bourhoods of Paris and Lisbon, could not have Join each bottom to its vessel by a hinge, AF, been constructed for want of a knowledge of so that it may open like the lid of a hox; and fluids rising to their common level, but probably let each bottom be kept up to its vessel by equal from the practical difficulty of uniting a long weights, BI, hung to lines which pass over the range of pipes, in such a manner as to remain pulley as at I, the blocks being fixed to the sides perfectly water-tight against the pressure of a of the vessel, and the lines tied to hooks at DB, heavy column of water, a circumstance which is fixed in the brass bottoms opposite to the hinges. by no means easy, even in our present state of Things being thus prepared, hold one vessel improvement, and with all the advantage of cast upright in the hand over a basin on a table, and iron and the most durable materials, instead of cause water to be poured slowly into it, till the stone or earthenware, which appears to have pressure of the water bears down its bottom at been chiefly resorted to for pipes in the formation the side, and raises the weight, and then part of of the older water-works.
the water will run out beneath. Mark the The New River water-works, which are of such height at which the surface of the water stood in, vast importance to the comfort and health of the the vessel when the bottom began to give way; great metropolis of England, are in themselves a and then, holding up the other vessel in the species of aqueduct, and unite all the varieties in same manner, cause water to be poured into it, the construction of water-works. The spring that and it will be seen that, when the water rises in supplies them rises at Ware, in Hertfordshire, and this vessel just as high as it did in the former, its waters are conducted in an artificial channel its bottom will also give way at the same height, or cut, formed for their conveyance alone, which and it will lose part of the water. is sometimes raised by arches and embankments The cause of this apparently surprising phevery considerably above the natural surface of nomenon is, that, since all the parts of a fluid at the ground, and at others sinks deeply into it, equal depths below the surface are equally for upwards of thirty-eight miles. At length it pressed in all directions, the water immediately ends in the open basin or reservoir, called the below the fixed part will be pressed as much New River Head, at Islington, which is suf- upward, against its lower surface within the vesficiently high to supply the lower parts of the sel, by the action of the column in the centre, as town by its natural descent into the pipes. To it would be by a column of the same height, accomplish the rest, a powerful steam-engine is and of any diameter whatever; and therefore, placed near this reservoir, for the purpose of since action and re-action are equal, and conworking pumps which force a part of the water trary to each other, the water immediately below into still more elevated reservoirs on Pentonville the surface, B, will be pressed as much downHi.l, and in the Ilampstead read, and what these wards by it as if it were immediately touched,
and pressed by a column of the whole height, sisting any very considerable pressure, Mr. and of the diameter AB; and therefore the Bramah suggested the use of a very strong metal water in the cavity beneath will be pressed as cylinder, in which a piston was so packed as to much downwards upon its bottom, F, as the bot move water-tight; and, as a substitute for the tom of the other vessel is pressed by all the high column of water, he employed a small water above it.
forcing pump, to which any power can be ap When a machine is constructed expressly, plied, and thus the pressing columu becomes for the purpose of showing, in the most striking indefinitely long, although the whole apparatus manner, that the pressure of Auids is as their is of itself comparatively small. perpend.cular heights, and that a quautity, how In plate I., fig. 6, we have a section of one ever small, may be made to support a weight, or of these presses, in which b is the piston of the another quantity, however large, it may be most large cylinder, formed of a solid piece of metal advantageously made in the form of what is turned truly cylindrical, and carrying the lower called the hydrostatical bellows. This apparatus board v of the press upon it: r is the piston of may now be examined. It consists of two çir- the small forcing pump, being also a cylinder of cular boards I, plate I. fig. 5, about sixteen solid metal, moved up and down by a handle inches in diameter; these boards are connected or lever. The whole lower part of the press is by means of a strong leather, which entirely sometimes made to stand in a case, Sa, containsurrounds them, and permits them to open and ing more than a sufficient quantity of water, as close like a pair of common bellows, with this at C, to fill both the cylinders; and the suction difference, that they open equally all round, and pipe of the forcing-pump dipping into this therefore the boards always remain parallel to water will be constantly supplied. Whenever, one another. The leather, at its junctures, is therefore, the handle is moved upwards, the well secured, and the whole machine is water water will rise through the covical metal valve, tight. In the upper board is fixed a pipe, AC, opening upwards into the bottom of the communicating with the interior, and reaching pump t; and, when the handle is depressed, above to a considerable height, suppose five feet. that water will be forced through another similar Through this pipe let some water he poured into valve g, opening in an opposite direction in the the bellows, and the upper board will be ob- pipe of a communication between the pump and served to rise a little; place a weight, B, upon the great cylinder d, which will now receive the it, pour in more water, and it will rise, though water by which the piston rod b will be elevated we increase the weight very considerably. In- at each stroke of the pump t. Another small deed, we may add water till the leathers are at conical valve, f, is applied hy means of a screw their utmost extension ; the water will then fill to an orifice in the lower part of the large cylinin the tube, and the upper board cannot be de- der, the use of which is to release the pressure pressed, nor the water forced out of the small whenever it may be necessary; for, on opening tube, until the pressure upon it is more than that this valve, any water which was previously conof a column of water whose diameter is equal tained in the large cylinder d will run off into to that of the interior of the bellows, and its the reservoir by the passage c, and the piston b height equal to that in the tube ; by increasing, will descend ; so that the same water may be therefore, the length of the tube, a most enor- used over and over again. The power of such mous weight might be raised by the pressure of a machine is enormously great; for supposing a few ounces of water.
the hand to be applied at the end of the handle To illustrate this singular experiment, we may with a force of only len pounds, and that this suppose a hole to be made in any part of the handle, or lever, be so constructed as to multiply upper board, and another tube to be inserted that force but five times, then the force with there; the water would certainly rise to the which the piston , descends will be equal to same level in them both; and, supposing the fifty pounds: let us next suppose that the magboard to be filled with tubes, the water would nitude of the piston b is such, that the area of obtain the same level in them all, because a its horizontal section shall contain a similar series of pipes would, in fact, form a solid area of the smaller piston r fifty times; then cylinder of water. If we suppose the hole to fifty times multiplied by fifty gives 2500 pounds, be of the same diameter as the interior of the for the force with which the piston b and the tube, and fitted with a piston, then, if the tube presser v will rise.
A man can,
however, contained two ounces of water, the piston would exert ten times this force for a short time, and sustain a weight of two ounces without being could therefore raise 25,000 pounds; and would depressed. If the area of the whole were twice do more if a greater disproportion existed bethat of the bore of the tube, two ounces in the tween the two pistons b and r, and the lever or tube would sustain four ounces on the piston. handle of the pump were made more favorable In this manner, every part equal to the bore of to the exertion of his strength. the tube is pressed upwards with a force equal Mr. Hawkins has contrived an hydrostatic to the weight of fluid in the tube. Hence, if weighing machine, which may be easily underthe proportion subsisting between the area of stood by reference to plate 1. fig. 7: а is a cythe tube and that of the bellows be multiplied linder made of rin, and japanned, which is by the weight of water in the tube, the product partly filled with water; b is another cylinder, will express the force with which the boards are rather less in diameter, resting upon and floatseparated.
ing in the water contained in the external vessel In lieu of the bellows part of the apparatus, A graduated scale and glass tube, c, are seen to rui the leather of which would be incanable of re- parallel to the vessel a, with which they are con