There are four species, two of which are natives of South America, two of the East Indies. HY'DROMANCY, n. s. Fr. hydromantie; Gr. ὕδωρ and μαντία. Prediction by water.

Divination was invented by the Persians: there are four kinds of divination; hydromancy, pyromancy, aeromancy, and geomancy.

Ayliffe. HY'DROMEL, n. s. Fr. hydromel; Gr. ὕδωρ and μέλι. Honey and water.

Hydromel is a drink prepared of honey, being one of the most pleasant and universal drinks the northern part of Europe affords, as well as one of the most ancient. Mortimer.

In fevers the aliments prescribed by Hippocrates were ptisans and cream of barley: hydromel, that is, honey and water, when there was no tendency to a delirium. Arbuthnot.

HYDROMETER, n. s. 2 Greek, vowp and

HYDROMETRY, N. s. } METPOV. An instru

ment to measure the extent or depth of water: the act of measuring it. HYDROPHOBIA, n. s. Fr. hydrophobie; Gr. ὑδροφοβία. Dread of water.

Hydrophobia is a kind of madness, well known in every village, which comes by the biting of a mad dog; or scratching [saith Aurelianus), touching, or smelling alone, sometimes (as Sckenkius proves), and is incident to many other creatures as well as man; so called because the parties so affected cannot endure the sight of water, or any liquor, supposing still they see a mad dog in it. Burton. Anat. Mel. HYDROPHOBIA in medicine, is a disease generally communicated to man by the bite of a rabid dog, and is so called because one of its principal symptoms is the inability of the patient to swallow water or any other liquid. It is called by some writers canine madness, and seldom makes its appearance till a considerable time after the bite of the rabid animal. In some few instances it has commenced in seven or eight days from the accident; but generally the patient continues in health for twenty, thirty, or forty days, or even much longer. The bite will in general be healed long before that time, frequently with the greatest ease; though sometimes it resists all kinds of healing applications, and forms a running ulcer, which discharges a quantity of matter for many days. The approach of the disease is known by the cicatrix of the wound becoming high, hard, and elevated, and by a peculiar sense of prickling at the part; pains shoot from it towards the throat; sometimes it is surrounded with livid or red streaks, and seems to be in a state of inflammation; though often there is nothing remarkable to be observed. The patient becomes melancholy, loves solitude, and feels sickness at the stomach. Sometimes the peculiar symptom, the dread of water, comes on all at once. Sometimes the disease begins like a common sore throat; and, the soreness daily increasing, the hydrophobic symptoms appear like a convulsive spasm of the muscles of the fauces. In others, the mind is first affected, and a real dread of water arises before the patient tries whether he can swallow it. But, in whatever manner this symptom comes on, the most painful sensations accompany every attempt to swallow liquids. Nay, the bare sight of water, or any thing clear, will give the utmost uneasi

ness, or even throw the patient into convulsions. The patient, however, is not as yet deprived of reason. Some have, merely by the dint of resolution, conquered the dread of water, though they never could overcome the convulsive motions which the contact of liquids occasioned, and yet this has been of no avail; for the convulsions and other symptoms increasing, have always overpowered the individual at last; and a great flow of viscid saliva into the mouth now takes place; as it has the same effect upon their fauces that other liquids have. This therefore is blown off with violence, which in a patient of Dr. Fothergill's occasioned a noise like the barking of a dog. Patients then have an insatiable thirst, but are unable to get down any drink without the utmost difficulty; though sometimes they can swallow bread soaked in liquids, slices of oranges, or other fruits. There is a pain under the scrobiculus cordis, as in the tetanus. But the symptoms are so various, that they cannot be enumerated; for we seldom read two cases of hydrophobia which do not differ very remarkably. Sometimes every member is convulsed by fits, but most violently from the navel up to the breast and œsophagus. The fit comes on perhaps every quarter of an hour, the fauces are not red, nor the tongue dry. The pulse is not at all feverish; and, when the fit is over, nearly like a sound pulse. The face grows pale, then brown, and during the fit almost black, the lips livid, the head is drowsy, and the ears tingling; the urine limpid. At last the patient is weary, the fits are less violent, the pulse becomes weak, intermittent, and not very quick; and at last the whole body becomes cold. If the patient can get sleep, so he will expire. The blood drawn a few hours before death appears good in every respect. The hydrophobia seems to be a symptom peculiar to the human race; for the mad animals which communicate the infection do not seem to have any dread of water.

With regard to the symptoms of madness in dogs, they are very equivocal; and those particularly enumerated by some authors are only such as might be expected in dogs much heated or agitated by being violently pursued and struck. The most certain symptom, indeed, is that all other dogs avoid and run away from one that is mad; and even large dogs will not attack one of the smallest size who is infected with this disease. You may discover whether a dog who has been killed was really mad or not, by rubbing a piece of meat along the inside of his mouth, and then offering it to a sound dog. If the latter eats it, it is a sign the dog was not mad; but, if the other rejects it with a kind of a howling noise, it is certain that he was. Dr. James tells us, that among dogs the disease is infectious by staying in the same place; and, that after a kennel has been once infected, the dogs put into it will be for a considerable time afterwards in danger of going mad. He rejects as false the opinion, that dogs, when going mad, will not bark: though he owns that there is a very considerable change in their bark which becomes hoarse and hollow.

It is said, that the causes commonly assigned for this disease among animals viz. heat, feeding

upon putrid flesh, want of water, &c., are not sufficient to produce the distemper. It does not appear that madness is more frequent among dogs in the warm than in the cold climates; may, in the island of Antigua, where the climate is very hot, and the water very scarce, this distemper has never, it is said, been observed. As to putrid aliment, it seems natural for dogs to prefer this to any other, and they have been known to subsist upon it for a long time without detriment. With regard to the immediate cause among mankind, there is not the least doubt that the hydrophobia is occasioned by the saliva of the mad animal being mixed with the blood. It does not appear that this can operate through the cuticula; but, when that is rubbed off, the smallest quantity is sufficient to communicate the disease, and a slight scratch with the teeth of a mad animal has been found as pernicious as a large wound. It is certain, also, that the infection has been communicated by the bites of dogs, cats, wolves, foxes, weasels, swine, and even cocks and hens, when in a state of madness.

If the disease once exhibits its symptoms in a human patient the chances for recovery are small indeed there having never been one well authenticated case of the recovery of a really hydrophobous person.-Prevention is the only chance, and removal of the contagious matter the only fair hope, of preserving life. Of all the means of removal, the cutting out the part to which the tooth has been applied, is unquestionably the most effectual. This therefore should not be delayed; one-quarter of an hour's hesitation will sometimes prove fatal. But, besides cutting away the part, careful washing may be used. Cold water should be poured upon the wound from a considerable height, that the matter may be washed away with some force. Even after removal by the knife careful washing is still proper. And after both these, to prevent, as far as can be, the possibility of any contagious matter lurking about the wounded part, it should not be allowed to heal, but a discharge of matter should be supported for several weeks, by ointment with cantharides, or similar applications. By these means there is the best chance of removing the matter at a sufficiently early period. Prevention may also be obtained by the destruction of the contagious matter at the part; and, where there is the least reason to think that a complete removal has not been obtained, these should always be had recourse to. With this intention the actual cautery, and burning with gun-powder, have been employed. And fire is, doubtless, one of the most powerful agents that can be used for this purpose. Recourse has also been had to washing, both with acids and alkalies. Of the former, vinegar has been chiefly used; but more may be expected from the latter, particularly from the caustic alkali, so far diluted that it can be applied with safety; for, from its influence as a solvent of animal mucus, it gives a better chance of a complete removal of the poison. See MEDICINE, Index.

HYDROPHYLUM, water leaf: a genus of the monogynia order, and pentandria class of plants: COR. campanulated, with five melliferous

longitudinal stria on the inside; the stigma is bifid: CAPS. globose and bivalved. There are three species, the chief is-H. Virginianum, the water leaf of Morinus. It grows naturally in Canada and many other parts of America on moist spongy ground. The root is composed of many strong fleshy fibres, from which arise many leaves with foot-stalks five or six inches long, jagged into three, five, or seven lobes, almost to the mid-rib, indented on their edges. The flowers are produced in loose clusters hanging downwards, are bell-shaped, and of a dirty white color. It may be propagated by parting the roots; which ought to be done in autumn, that the plants may be well rooted before spring, otherwise they will require a great deal of water. HYDROPICAL, adj. Fr. hydropique, HYDROPICK, adj. from Lat. hydrops;

Gr. vopomiкóg. Dropsical; diseased with extravasated water: resembling dropsy. Cantharides heat the watery parts of the body; as urine, and hydropical water. Bacon's Natural History.

The world's whole sap is sunk : The general balm the hydropick earth hath drunk. Donne.

Every lust is a kind of hydropick distemper, and the more we drink the more we shall thirst. Tillotson. Hydropick wretches by degrees decay, Growing the more, the more they waste away; By their own ruins they augmented lye, With thirst and heat amidst a deluge fry.

Blackmore.

Hydropical swellings, if they be pure, are pellucid. Wiseman. One sort of remedy he uses in dropsies, the water Arbuthnot. of the hydropicks.

HYDROSELENIC ACID. The best process which we can employ for procuring this acid, according to M. Berzelius, consists in treating the seleniuret of iron with the liquid muriatic acid. The acid gas evolved must be collected over mercury. As in this case a little of another gas, condensable neither by water nor alkaline solutions, appears, the best 'substance for obtaining absolutely pure hydroselenic acid would be seleniuret of potassium.

It

Seleniureted hydrogen gas is colorless. reddens litmus. Its density has not been determined by experiment. Its smell resembles, at first, that of sulphureted hydrogen gas; but the sensation soon changes, and another succeeds, which is at once pungent, astringent, and painful. The eyes become almost instantly red and inflamed, and the sense of smelling entirely disappears. A bubble of the size of a little pea is sufficient to produce these effects. Of all the bodies derived from the inorganic kingdom, seleniureted hydrogen is that which exercises the strongest action on the animal economy. Water dissolves this gas; but in what proportions is not known. This solution disturbs almost all the metallic solutions, producing black or brown precipitates, which assume, on rubbing with polished hæmatites, a metallic lustre. Zinc, manganese, and cerium, form exceptions. They yield flesh-colored precipitates, which appear to be hydro-seleniurets of the oxides, whilst the others, for the most part, are merely metallic seleniurets.

A human body forming in such a fluid, will never be reconciled to this hydrostatical law; there will be always something lighter beneath, and something heavier above; because bone, the heaviest in specie, will be ever in the midst. Bentley.

The weight of all bodies around the earth is ever proportional to the quantity of their matter: for instance, a pound weight, examined hydrostatically, doth always contain an equal quantity of solid mass. HYDROSTATICS and HYDRAULICS; from owo,

Id.

water, and saTIKN, the art of weighing and ύδωρ and ανλος, a pipe or fute, (the doctrine of machines worked by water, because organs when first invented were thus first sounded, or filled with wind). In the arrangement of our more important treatises we endeavour to unite the consideration of scientific propriety with the con venience of alphabetical order. Looking only to the latter of these, in regard to the above sciences, HYDRAULICS Would claim a prior place, but as this arrangement would, of necessity, tend to render the subject obscure and unconnected, it will be advisable to connect these important subjects and to treat of them in their natural order; beginning with the weight and pressure of water, and then showing the application of hydrostatic equilibrium in the construction of hydraulic machines.

We had better in the first instance explain the nature of fluidity. A perfect freedom of motion is essential to this state, and fluid bodies are usually divided into elastic, and non-elastic. Air is an example of the first, as its bulk may be lessened by augmenting the pressure, and enlarged by diminishing the compressive force. Water, on the contrary, is said to be non-elastic, or incompressible, not because it is absolutely so, but because its compressibility is so very small, as to make no sensible difference in our calculations, relative to its weight or motion.

The compressibility of water was a subject which engaged the attention of philosophers at a very early period. The Florentine academicians, from the following experiment, inferred that it could not be diminished in bulk: they took a globe of gold, which was the least porous of any body at that time known; and, having filled it with water, they closed it up. They then subjected the globe to a great compressive force, which squeezed the water through its pores, before any indentation could be made in it.

As a hollow sphere has a greater capacity than any other form, under the same surface, the academicians supposed that the compressive power, which was applied to the globe, must either force the particles of the fluid closer together, or drive them through the metal, before the globe yielded in the slightest degree to compression. With respect to its precise object, therefore, this celebrated experiment is not VOL. XI.

entirely conclusive, because they had no means of determining whether the diminution of the internal capacity of the globe by pressure, was exactly equal to the quantity of water forced through its pores; but they certainly proved the extreme minuteness of the particles that could be forced through so dense a metal as gold. The inference, drawn by the Florentines, remained uncontradicted till about 1762, when Canton published some experiments on the subject. With the barometer at 294°, and the thermometer at 50°, he states the following to be the results obtained ::

These results he obtained in the following manner: he took a glass tube, ahout two feet long, with a ball at one end, of an inch and a quarter in diameter; he filled the ball end part of the tube with water, which had previously been deprived of air as much as possible; he then placed it under the receiver of an airpump, and removed the pressure of the atmosphere: under this treatment, he observed that the water rose a little in the tube. On the contrary, when he placed the apparatus upon a condensing engine, and by condensing the air in the receiver, increased the pressure upon the water, he observed that the water descended in the tube. In this manner he proved that water expanded one part in 21,740, when the pressure of the atmosphere was removed; and submitted to a compression of one part in 10,870, under the weight of a double atmosphere. He also observed, that water possessed the remarkable property of being more compressible in winter than in summer; contrary to the effect on spirit of wine and oil of olives. Lest it might be supposed the compressibility, thus discovered, might be owing to air lodged within the fluids employed, a quantity of water was caused to imbibe more air than it contained in a preceding trial, but its incompressibility was not increased. There experiments, although, upon the whole, so apparently decisive of the questions they were instituted to determine, are not yet to be received without some caution; and, in particular, the remark that the addition of a portion of so compressible a fluid as air did not render water more compressible than before, is rather staggering; and is calculated to throw a veil of doubt over all the rest. It remained, therefore, for future investigators to determine the data for this branch of the science; but, even granting all the compressibility that has been contended for, the quantity of it is too small to be noticed in practice.

The piezometer employed by Mr. Perkins, in his experiments on the compressibility of water,

2 K

is represented in plate I. fig. 1. HYDROSTATICS and HYDRAULICS. The end I, of a cylinder S, three inches wide and eighteen long, being made water-tight by a plate firmly soldered to it: a cap, also water-tight, was screwed on the extremity. The rod a, ths of an inch in diameter, and carrying a flexible ring C, was made to pass through a tight stuffing-box F. The compression was effected in a cannon, the top of which was capable of containing the piczometer. It was fixed vertically in the earth, the touch-hole being plugged tight, and the muzzle about eighteen inches above ground. A strong cap was firmly screwed on at the mouth, and in the centre of it a small forcing pump, with a piston gths of an inch in diameter, was tightly screwed, and a valve introduced to ascertain the degree of pressure, one pound of pressure on that valve indicating an atmosphere. In performing experiments with this apparatus, the piezometer was introduced into the cannon, the water being forced in till the cap showed signs of leakage: the valve at the same time indicating a pressure of 100 atmospheres; when the piezometer was taken out of the cannon, the flexible ring, C, was eight inches upon the rod a, which proved that the rod had been forced that length into the cylinder, and that the compression was about one per cent.; in order to produce this compression, three per cent. must be pumped into the gun; an effect arising from the expansion of the gun, or the entrance of the water into the pores of the cast iron.

On his voyage to England, Mr. Perkins repeated this experiment frequently, and with the same result; by sinking the piezometer with fifty-four pounds of lead, to the depth of 500 fathoms, which gives nearly a pressure of 100 atmospheres. Being satisfied that the above piezometer would not show all the compression, he made another, consisting of a small tube, closed at the lower end, and water-tight; at the upper end, the water entered through a small aperture, closed by a delicate valve opening inwards; it was then perfectly filled with water (the weight of which was accurately known), and subjected in a common; hydraulic press to a pressure of about 326 atmospheres. When taken out, and weighed, there was found an increase of water, amounting to about 3 per cent. This water had been previously boiled and cooled down to 48°, and kept at that temperature during the experiment.

Fluids have weight, and gravitate towards the earth according to their density in the same way that solids do; but from the want of cohesion among their particles they are, however, incapable of assuming and retaining any particular form or figure without support and assistance, and consoquently they always take the form of the vessel which contains them, and they also exert a certain force against the sides of that vessel from their tendency to fall, which constitutes their lateral pressure; for fluids not only press downwards with their whole weight, in obedience to gravitation, but they press sideways or laterally in all directions at the same time and from the same cause, and consequently no fluid can reman in a state of quiet'equilibrium, unless every

part of its surface is equidistant from the centre of the earth, or in what is generally called a level plane, although that apparent plane is, in fact, not a plane, but partakes of the convexity of the earth. And it is for the purpose of establishing such an equilibrium that fluids always run from a higher to a lower situation.

For the purpose of explaining the manner in which the surfaces of fluids become level, it may be very fairly supposed that the particles of which they are composed are placed one upon another so as to form what may be termed pillars or columns of particles as represented in plate I. fig. 2, and supposing all the particles to be of the same size and weight, then the six which are on one side will be an exact balance to the six which are on the other (both columns being supported by the bottom of the vessel which contains them), and their two tops will be level; but, if the two upper particles t and v are supposed to be taken away, a balance can no longer exist, for now there will be six particles in one column, while there are but four in the opposite one to press against and resist them, the consequence of which will be that the tallest column will descend, and the particle u will fall into the situation of w, while that marked a will, with its column, ascend into the situation e, and thus a and u will come to the same level, and a balance or equilibrium will be again restored. Every vessel is supposed to be filled by an infinite number of such columns, although two only are represented in the figure to prevent confusion. The cause of bodies floating upon fluids, or sinking in them, may be explained by the same reasoning, for whenever a solid is immersed in a fluid, it displaces a quantity of water, and consequently renders the columns of particles underneath it shorter, and therefore lighter, than those which surround it. It will only then be to conceive that the two particles t and e, in the last figure represent a body which is partly immersed in the water, and is floating near its surface; the columns under that body will be shorter than those which surround it, but the weight of the body becomes a counterpoise to the greater length of the surrounding columus, and must in every case be precisely equal to the quantity of water which it displaces, otherwise it cannot float, for all bodies which are incapable of so becoming this counterpoise, or, in other words, are so heavy that their small bulk will not permit them to displace as much water as is equal to their own weight, must inevitably sink; consequently all things which are lighter than their bulks of water will swim, and all that are heavier must sink, unless when they are placed in a boat or hollow vessel, which by its bulk enables them to displace more water than is equal to their weight, and then they will float. A ship therefore of 500 tons burthen must displace 500 tons of water from the bed or hollow which it makes itself up to its water-line, and in this way the tonnage of vessels is estimated.

The truth of this position is very satisfactorily proved by putting the model of a ship or any other body capable of floating into a scale, and exactly balancing it with water in the other scale. The floating body is then to be removed and

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 atWare, 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 D B, 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,