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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 centre 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 proportionably 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 runnig 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 circumfer. ence 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 [..."; fins, from which rectangular uckets 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 HYDRAU. Lics 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, op 9, with their pistons, as acted upon by three cranks, a b c, each equally radiated from the branch de, and moved by a water wheel, of which fis the axis: it is plain that the several cranks stand at an angle of 120 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 acelerate 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 applying such a spur, &c. see the article Mill-Work. HYDRAULICON, water-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 extremely troubler some to the patient. See SURGERY, HYDROCEPHALUS, insurgery, a preternatural distention of the head to an uncommon size, by a stagnation and extravasation of the lymph, which, when collected withinside of the bones of the cranium, the hydrocephalus is then termed internal; as it is external, when retained between the common integuments and the cranium. See MEDic1NE. HYDROCHARIS, in botany, a genus of the Dioecia Enneandria class and order. Natural order of Palmae. Hydrocharides, Jussieu. Essential character: male, spathe two-leaved; calyx, trifid ; corolla, three petalled; filaments the three inner style bearing: female, calyx trifid: corolla three petalled ; style six; capsule six-celled, many seeded, inferior. There is but one species, with many varieties, viz. H. morsus ranae, frog bit. HYDROCOTYLE, in botany, marsh pennywort, a genus of the Pentandria Digynia class and order. Natural order of Umbellatae. Essential character: umbel simple, with a four-leaved involucre ; petals entire; seeds semi-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 Fluids, 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. Van Helmont gave this the name of gas igneum, 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 atmospheric air, exploded when kindled by a match. Mr. Cavendish, however, first examined its properties fully, show d that it is permanently elastic, not absorbed by water, and that it is much lighter than atmospheric air. (Philos. Trans. vol. lvi. 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 the 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 ef. fected 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 earthen 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 decomposed, 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 es: capes, 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 suf. fers 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 expense 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. Its specific ...} in this state, water being 1000, is stated b Lavoisier at 0.0946.100 cubic inches weig 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 burn 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, 8vo. vol. i. p. 129; ibid. vol. iv. p. 23). Though hydrogen gas beinflammable,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. Scheele long ago observed, that he was able to breathe it for twenty inspirations. (Treatise on Air and Fire, p. 160.) Fontana showed, what Scheeleindeed had observed, that if the lungs were previously emptied as much as possible of atmospheric air, by a forcible expiration, it cannot be breathed so long, though still it did not appear to him to be positively deleterious, like some of the unrespirable gases, (Opuscules Physiques, p 2.) 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. 456.) 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 mourishment 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 less 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 maarshy situations, from the slow decomposition of vegetable and animal substances, holding, dissolved in it, carbon, and P. also phosphorus and nitrogen, and orming, 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 atmo

sphere, 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, gulphs, creeks, &c.; 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 cordate at the base; capsule two-celled, two-valved. There are four species. HYDROMANCY, a method of divination by water, practised by the ancients in this manner; they filled a cup or bowl of water; then fastening a ring to a piece of thread tied to their finger, held it over the water, and repeated a certain form of words, desiring to be satisfied with regard to their inquiry; and if the question was answered in the affirmative, the ring would strike the sides of the bowl of its own accord. HYDROMETER. The best method of weighing equal quantities of corrosive volatile fluids, to determine their specific gravities, appears to consist in inclosing them in a bottle with a conical stopper, in the side of which stopper a fine mark is cut with a file. The fluid being poured into the bottle, it is easy to put in the stopper, because the redundant fluid escapes through the notch, or mark, and may be carefully wiped off. Equal bulks of water and other fluids are by this means weighed to a great degree of accuracy, care being taken to keep the temperature as equal as possible, by avoiding any contact of the bottle with the hand or otherwise. The bottle itselfshews, with much precision, by a rise or fall of the liquid in the notch of the stopper, whether any such change has taken place. See GRAv1Ty, specific. But as the operation of weighing requires considerable attention and steadiness, and also a good balance, the floating instrument, called the hydrometer, has always been essteemed by philosophers, as well as men of business. It consists of a hollow ball, either of metal or glass, ca}. of floating in any known liquid ; rom the one side of the ball proceeds a stem, which terminates in a weight, and from the side diametrically opposite proceeds another stem, most commonly of an equal thickness throughout. The weight is so proportioned, that the instrument may float with the last mentioned stem upright. In the less accurate hydrometer this stem is graduated, and serves to show the density of the fluid, by the depth to which it sinks, as the heavier fluids will buoy up the instrument more than such as are lighter. In this way, however, it is clear, that the stem must be comparatively thick, in order to possess any extensive range; for the weight of vitriolic ether is not equal to three-fourths of the same bulk of water, and therefore such an hydrometer, intended to exhibit the comparative densities of these fluids, must have its stem equal in bulk to more than one-fourth of the whole instrument. If this bulk be given chiefly in thickness, the smaller dif. ferences of density will not be perceptible, and it cannot, with any convenience, be given in length. To remedy this imperfection, various contrivances have been proposed, for the most part grounded on the consideration, that a change in the ballast, or weight employed to sink the ball, would so far change the instrument, that the same short range of gradations on a slender stem, which were employed to exhibit the densities of ardent spirits, might be employed in ... experiments upon water. Some have adjusted weights to be screwed upon the lower stem. and others, with more neatness and accuracy, have adjusted them to be slipped upon the extremity of the upper stem. But the method of Fahrenheit appears to be on all accounts the simplest and most accurate. The hydrometer of Fahrenheit consists of a hollow ball, with a counterpoise below, and a very slender stem above, ter. minating in a small dish. The middle, or half length of the stem, is distinguished by a fine line across. In this instrument every division of the stem is rejected, and it is immersed in all experiments to the middle of the stem, by placing proper weights in the little dish above. Then, as the part immersed is constantly of the same magnitude, and the whole weight of

the hydrometeris known, this last weight, added to the weights in the dish, will be equal to the weight of fluid displaced by the instrument, as all writers on hydrostatics prove. And accordingly the specific gravities for the common form of the tables will be had by the proportion. As the whole weight of the hydrometer and its load, when adjusted in distilled water, is to the number 1,000, &c. so is the whole weight, when adjusted in any other fluid, to the number expressing its specific gravity. in order to show the degree of accuracy an instrument of this kind is capable of, it may in the first place be observed, that the greatest impediment to its sensibility arises from the attraction or repulsion between the surface of the fluid and that of the stem. If the instrument be carefully wiped with a clean soft linen cloth, the metallic surface will be equally disposed to attract or repel the fluid. So that if it possess a tendency to descend, there will be a cavity surrounding the stem; or if, on the contrary, its tendency be to rise, the fluid will stand round the stem in a small protuberance. The operator must assist this tendency by applying the pincers, with which he takes up his weights to the rim of the dish. It is very easy to know when the surface of the fluid is truly flat, by observing the refleced image of the window, or any other fit object seen near the stem in the fluid. In this way the adjustment of the weights in the dish may, without difficulty, be brought to the fiftieth part of a grain. If, therefore, the instrument displace one thousand grains of water, the result will be very true to four places of figures, or even to five. This will be as exact as most scales are capable of affording Some writers have spoken of the ad

justment of an hydrometer of this kind,

so that it shall at some certain temperature displace one thousand grains of water, as if this were a great difficulty. It is true, indeed, that the performance of a piece of workmanship of this nature would require both skill and judgment on the part of the artist; but it is by no means necessary. Nothing more is required on the part of the workman, than that the hydrometer shall be light enough to float in ether, and capable of sustaining at least onethird ...}. own weight in the dish, without oversetting in a denser fluid. This last requisite is obtained by giving a due

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