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Ill this contrivance the piston, A, see fig. 9, has a valve which, as the rod draws up, is closed by the pressure of the air above it; but in descending it opens, and allows the water, which had Mowed into the lower part, whence the air was withdrawn, to rush through ; as the piston is raised again, the weight of the water forcibly oppresses the valve, until it find a lateral passage at B, whence it issues, and in this manner any quantity may be raised. If the water has a direct issue, as in the common spouts of pumps, no further apparatus is wanted, but if it is to be retained, or pass through any other pipes more elevated than the debouchure, B, there must be a small angular projection, as shown by the dotted lines, to admit the valve C, also pointing upwards. In dry weather, or when the pump is not much rsed, the leather binding of the piston, as also the valves, will become dry; therefore it is necessary, on such occasions, to throw in a pail-full or two of water to moisten them; else the air will pass downwards as the piston rises, and prevent that exhaustion on which the ascent of the water depends. It is generally necessary to have a valve at the bottom of the pipe to keep in the water drawn into it, in order that the labour may be decreased; and that, if the pumping be intermitted, there may be Us* trouble in bringing up the water within reach of the piston.

Where the water lays near the surface, a lifting-pump may be used. This is nearly the same with the former; but requires the piston should be forced down beneath the level of the water in the well. In this it is not so indispensably necessary that the leather on the piston should fit so close: though it is the better for so doing. In the liftingpump the whole depends on actually raising the water from the well as though it were done by means of a bucket, this occasions many to apply that designation to the piston. The same precautions are necessary if the water is to be passed into any pipe, as have been stated regarding the debouchure of the sucking-pump.

The forcing-pump has a solid piston, as Men at A in fig. 10, which, after the water has pasted the valve at B, is pressed down, and causes the fluid to pass into the conducting pipe, C, where there is also a valve, d, to prevent its return. The valve at B closes at the piston descends, while that at d rises to allow its escape from the main pipe. When the piston rises, the water follows, as in the two former instances, through

VOL. III.

the lower valve B, while the smaller valve at d is also closed by the super-incumbent water in the conduit, e, and by the attraction of the piston, the water rushing after it to prevent a vacuum. In this kind of pump the piston must fit extremely close ; both on account of the intended attraction of the fluid from below, and to prevent its escape upwards when the piston is pressed downwards.

The whole of those inventions which raise water by alternate risings and fallings of only one piston are subject to the inconvenience of having the water issue in jerks, which, in some instances, would prove highly inconvenient. To remedy this, a cistern should be placed near the debouchure, or spout, whence a small stream would flow with much less variation than from the spout itself. But the best mode of regulating the issue of water is by aid of an air-vessel, as in a fire-engine. See Pneumatics.

To detail all the varieties of pumps that arc in use would be both beyond the limits of this work, and of no real utility to the reader: we shall therefore enter upon the description of the valves in general estimation, and then proceed to give a brief account of hydraulic machinery.

The most common kind of valve consists of a piece of stiff leather, such as is applied for soles in shoes, and is generally known by the name of pump-leather. On its upper side a piece of milled lead is rrvctted firmly, and the part where it is to be fixed on the frame, or shell, of the piston, is grooved for the purpose of giving it pliancy, that it may work up and down as if on a hinge. Fig. 11, shows the plan; and fig. it, the profile of this valve, which is cheap, simple, and easily repaired, though it has the defect of being liable to choke, and of not rising high enough to allow a sufficient passage for the water.

Fig. 13, shows a button*alte, which is merely a piece of turned metal, A, having a shank, B, of about eight inches or a foot in length, according to the depth of the block, x x. The shank passes through the bar, C, at the bottom of the block, and is prevented from coming up too high by the stud, or nut, «, at its bottom. When the water rises, it forces up the button. A, and passes through the hollow in the block, of which the superior part is expanded so as to fit the button, which being the frustrum of a cone, necessarily fits close into the expanded part as the water presses it, after having passed upwards in consequence of the descent of LI

the piston; which may either be solid, as in a forcing-pump, or valvcd, as in a lifting or a sucking pump. This valve may be applied to a piston, as well as to that part of the pipe which retains the water, that it may be within reach of the piston's action. An improvement has been made to this valve, by adding a ball of some weight to the bottom of the shank, B, and excavating the button, in order to reduce its weight in proportion: this insures the regular descent of the button to its seat.

The butterfly-valve, exhibited in fig. 14, varies from the two former, in having two semicircular flaps appended by hinges to a bar passing over the centre of the excavated piston. This valve is peculiarly eligible, because if one part should be stiff, and adhere to the piston, the other will play with an increased effect, though not equal to the action of both valves.

The simplest valve with which we are acquainted is the sphere, which is made of metal, and fits into a semi-spherical cavity on the top of the piston or block. When the piston (if it be on that) rises, the sphere falls into the socket; but when the piston is depressed, the rush of water from below forces the sphere upwards. The only inconvenience attendant upon this valve,which is shown at fig. 15, is, that its diameter being nearly equal to that of the bore, leaves a very narrow passage for the water. This, however, might perhaps be obviated, by making an excavation in the pipe, as shown by the dotted lines, and by driving nails through to obstruct the ball from rising too high.

These are the general principles of the valves in common use; though we could enumerate a great variety, which have all been strongly recommended, but in practice proved very deficient. We shall therefore proceed with the detail of hydraulic machines, commencing with those which supply the place of pumps, by raising water to given heights. The most simple, and perhaps the most ancient, is the spiral pump of Archimedes. It consists of a cylinder of wood, about a foot in diameter, and of any length at pleasure: on this a leaden pipe of any bore is wound from the bottom to the top, spirally. When the bottom of the cylinder revolves in the water, (by means of a common winch handle at the top, and of a pintle in the t'entn: of its base, which rests in a box or step for that purpose below) the reclined position, as shown in fig. 16, occasions the water to enter the bottom of

the pipe, and to be carried by the revolutions of the cylinder completely up to the top, where it discharges into a vessel. This, however, raises but a small quantity, though the height may be indefinite: therefore, where such a machine is in use, it will be found eligible to have the whole cylinder covered with various pipes, like the bands in a rope, whereby the quantity of water raised would be proportionality increased, with very little addition of power: the greatest resistance would arise from the friction upon the supporting axis, especially the lower one under the surface. Some of these machines have been worked in strong running brooks, by means of water-boards, the same as the great wheels in undershot mills.

The horn-drum, so called from a number of segments passing from the circumference of a large flat cylinder to its centre, is an easy mode of raising water. The scoops, or mouths, by turns dip into the water, and as they rise cause it to pass up the horn, or segment, until it is discharged into a trough placed under the end of the axis, which is hollow, and has its pintle fastened to a cross, as seen in fig. 17. Such wheels usually work with water (or float) boards; and some of them have projecting fins, from which rectangular buckets are suspended: these dip into the water as the wheel turns, and successively discharge into a trough, by means of a pin at A, which causes every bucket as it passes to turn to a horizontal instead of an erect position. The latter invention is ascribed to the Persians. The reader will, no doubt, readily perceive that a strong current, or other force, is needful to move machines so laden as the Persian wheel, it sometimes raising near a ton of water in each revolution ; and that nothing but the necessity for raising water could induce to so great a loss of power. When treating of Mills and of Pumps, as also of PneuMatics, with which Hydraulics are often intimately blended, we shall enlarge more on this subject; for the present concluding with the ordinary mode of applying a water wheel to pumps, as may be seen at London Bridge, and in a great variety of instances, where immense quantities are raised by means of running water, referring to the article Steam-engine for the operations dependant on that power. We have, in speaking of Fluids, said much on their properties, which the reader will find both amusing and instructive: indeed we consider this doctrine to be indispensable, as

a study, with those who court an intimate acquaintance with hydraulics.

Fig. 18, shows the section of three forcing pumps, o ji q, with their pistons, as acted upon by three cranks, a b c, each equally radiated from the branch d e, and moved by a water wheel, of which/ is the axis: it is plain that the several cranks stand at an angle of 1 JO degrees respectively. By this means there is a counterbalance among them mutually, and each gives one stroke or plunge during each revolution of the wheel. If the wheel is large, it will of course move slowly; and, unless the pumps be very large, but little water will be raised: therefore it is usual to accelerate the motion of the branch bearing the cranks, by means of a spur, or of a trundle, turned by the water-wheel, and bearing such proportion thereto as the required increase of velocity may demand. For the manner of apply ing such a spur, &c. see the article MillWork.

HYDRAULICON, tcater organ, in music, an instrument acted upon by water, the invention of which is said to be of higher antiquity than that of the wind organ.

HYDROCELE, in surgery, denotes any hernia arising from water, but is particularly used for such a one of the scrotum, which sometimes grows to the size of one's head, without pain, but exceeding troublesome to the patient. See Surgery.

HYDROCEPHALUS, in surgery, a preternatural distention of the head to an uncommon size, by a stagnation and extravasation of the lymph, which, when collected within side of the bones of the craninm, the hydrocephalus is then termed internal; as it is external, when retained between the common integuments and the cranium. See Medicine.

HYDROCHARIS, in botany, a genus of the Dioecia Enneandria class and order. Natural order of Palms. Hydrocharides, Jussieu. Essential character : male, spathe two-leaved; ' calyx, trifid; corolla, threepetalled; filaments the three inner style bearing: female, calyx trifid; corolla three petalled; styles six; capsule six-celled, many seeded, inferior. There is but one species, with many varieties, viz. H. morons rana\ frog bit.

HYDROCOTYLE, in botany, marsh nyicort, a genus of the Pentandria Digyclass and order. Natural order of I'm,tje. Essential character: umbel sim'pi*',' with a four-leaved involucre; petals

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entire; seeds lemi-orbiculate, compressed. There are fifteen species.

HYDRODYNAMICS treat of the powers, forces, and velocities, of fluids in motion. Having entered fully into the detail of all relating thereto while treating of Fluid, Hydraulics, Hydrostatics, Mills, and Water wheels, we forbear from that repetition which would trespass on the space allotted to other articles, referring the reader to those heads for what appertains thereto.

HYDROGEN. It had been long known to the chemists, that a vapour or air is disengaged in some processes, which kindled on the approach of an ignited body. Vau Helmont gave this the name of gas igneura, and it seems to have attracted the attention of Boyle, Mayow, and Hales. The chemists knew, that such a vapour or air was commonly disengaged during the solution of certain metals, in muriatic or dilute sulphuric acid, that it burnt at the mouth of the phial, and if mixed with atmosphericair, exploded when kindled by a match.

Mr. Cavendish, however, first examined its properties fully, shewed that it is permanently elastic, not absorbed by water, and that it is much lighter than atmospheric air. (Philos. Trans, vol. Ivi. p. 141). This substance forming water when combined with oxygen, and being therefore the radical of that compound, the name hydrogen was given to it, at the formation of the new nomenclature.

It is always obtained from tire decomposition of water, as it cannot, from other substances in which it exists, be easily disengaged in perfect purity. Some substance is made to act on water, which exerts an attraction to the oxygen, without combining with the hydrogen, when, of course, the hydrogen is disengaged, and passes into the elastic form.

At the common temperature of the globe, this decomposition cannot be effected with rapidity by any single affinity. Iron, moistened with water, decomposes it very slowly, and evolves hydrogen; but at the temperature of ignition, the decomposition is more rapid. If a coil of iron wire, or a quantity of iron filings be put into an iron or coated glass, or earthern tube, which is placed across a small furnace, and surrounded with burning fuel, so as to be brought to a red heat, on distilling water from a retort connected with it, the vapour, in passing over the surface of the ignited iron, is decouiLlfl

posed, the iron attracts its oxygen, and hydrogen gas issues from the extremity of the tube.

This process is a troublesome one, and by the agency of an acid, water is decomposed as rapidly by iron or zinc, at a natural temperature. Zinc affords the hydrogen in the greatest purity. One part of it, in small pieces, is put into a retort, or a bottle with a bent tube adapted to it; two parts of sulphuric acid, previously diluted with five times its weight of water, are poured upon it, an effervescence is immediately excited, hydrogen gas escapes, and is to be collected in jars filled with water, and placed on the shelf of the pneumatic trough. Its disengagement continues until the zinc is dissolved. Iron may be employed in place of zinc, but containing generally a little carbon, which is dissolved by the hydrogen, it affords a gas less pure. Muriatic acid serves the same purpose as sulphuric acid, but must be diluted with only twice or three times its weight of water.

In the experiment, the hydrogen gas is derived entirely from the decomposition of the water, the oxygen of which is attracted by the metal. That the acid suffers no decomposition, is proved by the liquor at the end of the experiment, being capable of saturating as much of an alkali as the quantity of acid employed would have done in a pure state. The agency of the acid was formerly explained, on the absurd doctrine of disposing affinity,—that it had no attraction to the pure metal, but to the oxide of the metal; that to satisfy this affinity, it caused the oxidation of the metal at the expence of the water, and then combined with the oxide thus formed. In conformity to Berthollet's speculations, it may be referred to the affinities of the acid to iron, and to oxygen, conspiring with the affinity of iron to oxygen: these, co-operating, produce a ternary combination, while the hydrogen gas is disengaged.

Hydrogen gas is permanently elastic. When collected over water, it is observed to have a peculiar smell, slightly fetid, Which is not so perceptible when it is collected over quicksilver, and which is lost when the gas is exposed to substances which powerfully attract humidity. It is not the only substance in which water appears requisite to develope odour.

This is the lightest of the gases, and indeed the lightest substance whose gravity

can be ascertained by weighing. Its specific gravity varies considerably, according to its state with regard to humidity. When it has been transmitted through water, or has remained for some time exposed to it, it is about ten times lighter than atmospheric air; when it has been received over quicksilver, and exposed to any substance which attracts water strongly, as quicklime, it is nearly 13 times lighter, or atmospheric air being 1,000, hydrogen is 84. Ita specific gravity in this state, water being 1,000, is stated by Lavoisier at 0.O946. 100 c ubic inches weigh 2.613 grains. It is from this levity, that it was applied with success to the construction of balloons ; a varnished silk or linen bag, filled with it, having a specific gravity so much less than atmospheric air, as not only to rise in the atmosphere, but also to elevate an additional weight

The chemical property by which hydrogen gas is most eminently distinguished, is its great inflammability. When an ignited body is approached to it, in contact with the atmosphere, it is immediately kindled, and continues to bum while the air is admitted ; if previously mixed with atmospheric air, and a burning body approached to the mixture, or an electric spark sent through it, it instantly inflames with detonation; and when it has been mixed with oxygen gas, the detonation is more violent. When burning at the extremity of a capillary tube, on bringing a wide tube over the flame, a singular phenomenon, accidentally' observed by Dr. Higgins, is produced, that of sounds of various tones, which vary in acuteness and strength, according to the width, the length of the tube, and the kind of substance of which it is formed, owing, apparently, as Picket and De la Rive have explained it, to the vibrations excited in the matter of the tube by the rapid expansion and condensation of the watery vapour near and around the flame, and which, regulated and equalized by regular reflections from the sides of the tube, constitute a musical sound. (Nicholson's Journal, Bvo. vol. i. p. 129; ibid, vol. iv. p. 23).

Though hydrogen gas be inflammable, it is incapable of supporting the combustion of other inflammables. If a burning body be quickly immersed in it, it is immediately extinguished.

This gas is incapable of supporting animal life by respiration; an animal immersed in it is soon killed. At the same time, it does not appear to be so positively deleterious as the other noxious gases. Schcele long suiO observed, that he was able to breathe it for twenty inspirations. (Treatise On Air and Fire, p. 160). Fontana shewed, what Scheele indeed had observed, that if the longs were previously emptied as much as possible of atmospheric air, by a forcible expiration, it cannot be breathed so lone, though still it did not appear to him to be positively deleterious, like some of the unrcspirable gases, (Opuscules Physiques, p. 3). Rosier, even after expelling the air from the lungs, breathed hydrogen gas for several respirations ; and Mr. Davy, in his experiments on the respiration of the gases, remarked, that in one experiment, after a complete exhaustion of the lungs, he found great difficulty in breathing hydrogen for half a minute, though in a subsequent experiment, with the same preparation, he breathed it for near a minute. The first six or seven inspirations produced no sensations whatever; in half a minute, a sense of oppression was felt at the breast, which increased until the pain of suffocation interrupted the experiment. (Chemical Researches, p. 400. 466). Hydrogen, therefore, is incapable of supporting life ; the respiration of it can be continued only for a short time, and animals confined in it soon die. It appears only to prove fatal, not by a positively noxious quality, but by excluding atmospheric air, the due supply of which, by respiration, is indispensable to life. Blood exposed to it acquires a deep black colour, and the gas suffers a diminution of volume.

Hydrogen is not, as several of the other gases are, noxious to vegetable life; at the same time, it appears to contribute little to the nourishment of plants. Dr. Priestley having found, that it still continued inflammable after a growing vegetable had been confined in it for several months. It can apparently supply, to a certain extent, the place of light, in supporting vegetation. Von Humboldt observed, that some cryptogamic plants in mines, and of course secluded from light, were not pale, but of a green colour, such as they would have had from growing under exposure to the light of day; and he concluded, with sufficient probability, that the agency of light had, in this case, been supplied by the hydrogen gas, which is evolved in greater or leu abundance in such situations.

Hydrogen gas is so sparingly soluble in water, that when agitated with it, it suffers

no perceptible diminution of volume. When the water has been previously freed from atmospheric air, Mr. Henry found, that one hundred cubic inches take up 1.5 of the gas under a common atmospheric pressure; under increased pressure, a larger quantity, equal to one-third of the volume of the water, is absorbed.

The affinities of hydrogen seem principally exerted towards inflammable bodies. It unites with sulphur, phosphorus, and carbon, and forms gaseous compounds; it appears to be capable of dissolving even some of the metals, particularly iron, zinc, and arsenic. United with nitrogen, it forms one of the alkalies, ammonia; with oxygen, water. It is also a constituent principle of the greater number of the vegetable and animal products.

Hydrogen gas may be regarded as a product of some natural operations. It is found collected often in mines, derived probably from the decomposition of water by metals; it is known to the miners by the name of fire-damp, and is often the cause of accidents, from exploding on the approach of an ignited body. It is also extricated from stagnant water, and from marshy situations, from the slow decomposition of vegetable and animal substances holding, dissolved in it, carbon, and perhaps also phosphorus and nitrogen, and forming, as has been supposed with some probability, gases which render the air of such places unhealthy. From its levity, it has been supposed that the quantity of it thus produced at the surface of the earth, will rise through the atmosphere, and occupy the higher regions; and on its presence, some of the phenomena of meteorology, particularly the sudden appearance of some fiery meteors, have been supposed to depend. Its affinities have not been ascertained with any precision, as to their relative force.

HYDROGRAPHY, the art of measuring and describing the sea, rivers, lakes, and canals. With regard to the sea, it gives an account of its tides, counter-tides, soundings, bays, gulpbs, creeks, Sec.; as also of the rocks, shelves, sands, shallows, promontories, harbours, the distance and bearing of one port from another, with every thing that is remarkable, whether out at sea, or on the coast.

HYDROLEA, in botany, a genus of the Pentandria Digynia class and order. Natural order of Convolvuli, Jussieu. Essential character: calyx five-leaved; corolla wheel-shaped; filaments cordite at the

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