bodies moving through them. If an orifice in a vessel present downward, and the column of liquid over it be short, this will simply drop out by its own weight, starting at a velocity of 0. But if a considerable depth of liquid be above, its gravity produces a corresponding pressure on its base, or on that liquid which is near it; so that, if a plug be removed from an orifice in or close to the base, the liquid starts at once into rapid motion. Theoretically, the velocity with which it thus starts to move is equal to that which a body would acquire in falling freely, in vacuo, from the surface of the liquid to the centre of the orifice. If this depth be 16 feet, the initial velocity will be 32 feet per second, and so on. The velocity is independent of the density of the liquid, being determined solely by the depth below the surface; and it is proportional to the square root of the depth, being 5 times as rapid at a depth of 25 feet as at 1 foot. If the liquid in the vessel be kept at the same level, the quantity discharged in a given time will be in theory equal to the product of the area of the cross section of the escaping jet at the orifice into the length of jet delivered. The time required by a vessel to empty itself is to the time required, when it is kept constantly full, to discharge the same quantity of water, as 2 to 1; and the spaces described by the surface in its descent in a column of equal size throughout, are as the odd numbers, *** 9, 7, 5, 3, 1. Thus, these spaces measure equal times, as in the clepsydra, or water clock. Since liquids are not perfectly mobile, and their exit at an orifice must be retarded by cohesion and friction, the results thus far given are much modified in practice. When a liquid flows through an orifice in a vessel, eddies are formed about the sides of the orifice, preventing the escape of a jet equivalent to its full size; and owing to these, and to acceleration of velocity, if the jet be downward, it rapidly contracts in its diameter. At a distance outside about equal to the diameter of the orifice, it is contracted to or its area at first; and this part has been called the "contracted vein." Savart has shown that below this the stream still contracts, though less rapidly. Through a certain length the jet seems to remain entire and pellucid; but it soon becomes turbid, being broken into drops in a series of layers. Savart also found that these alternately flatten laterally and elongate, forming what he has called ventral segments and nodes. These layers separate more widely as they descend with increased rapidity; but falling through great heights, the whole may finally be dissipated in a mist. The formation of the drops doubtless takes place in consequence of pulsations in the liquid, originating at the moment of its passage through the orifice. In this view the most recent and careful investigator of the subject, Plateau ("Philosophical Magazine," Oct. 1856), essentially coincides. A musical sound is produced at the orifice, and undoubtedly by the action of molecular forces similar

to those which determine the "singing" of burning jets of gas. Owing to disturbing influences such as those now named, the actual efflux of a liquid is always much less than the theoretical. Through a thin wall not more than 15 feet in depth, the former is only about 64 parts in 100 of the latter. A short pipe inserted so as to reach within the orifice diminishes the flow; not reaching within, and having a length of twice its diameter, it increases the efflux to 82 parts; having the form of the contracted vein, to 95 parts; and when the sides of the vessel approach the tube within in paraboloid form, and the tube itself is trumpetshaped, the discharge may equal or exceed that required by theory. Liquid jets from the sides of vessels may be considered as discharges of continuous bodies of projectiles (drops) slightly held together by cohesion. In practice, such a jet directed upward falls considerably short of reaching the height of the liquid surface in the vessel or reservoir. When the jet is directed horizontally it tends, but for the resistance of the air, to describe a curve in form of a parabola. All the results now stated are further modified in those semi-liquids which have greater or less viscidity, as sirup, fixed oils, &c.-In theory, cylindrical tubes of the same length, having their diameters as 2 to 1, should deliver in the same time quantities of liquid that are as 4 to 1; and generally, the quantities will be as the squares of the diameters. But owing to the fact that the friction is relatively greater the smaller the tube, it results in practice that two tubes will not deliver quantities that are as those squares, but the larger will afford proportionally more. Under the same conditions, a tube 70 yards long of the same size with another that is 1 inch long, will deliver only about the quantity of liquid; two pipes of 2 and 1 inch diameter, 100 feet long, yield quantities as 5 to 1, not as 4 to 1; and to secure a given discharge by a long pipe, its diameter must usually be greater than that required by theory. In tubes of unequal size in different parts, and kept filled, the flow is somewhat quickened at the contracted portions so as in a degree to compensate for loss of capacity; if any portion is greatly enlarged, eddies are formed in it, and the flow is retarded; as it is also by greatly diminishing the caliber, as is done by cocks, and in aëriform fluids by dampers. If a single pipe terminate in several smaller ones, the velocity in the smaller is less, unless their collective area be made as much larger than that of the single one as the friction arising within them is greater; a principle well illustrated in the case of arteries and capillaries in the animal body. And any pipe will deliver in a given time a quantity of liquid that is greater in proportion as its inner surface is smooth; as the liquid is one having less adhesion for the pipe; as the head of water and rapidity of descent of the pipe are increased; as the bore is more nearly uniform; and as the curves in its course are more gradual, or its direction approaches a straight line, Singularly, too, even

with long tubes the amount delivered is increased by shaping the exit of the reservoir into the tube in form of a paraboloid, and still more by terminating the tube in a conical opening, enlarging outward. If a current of water be sent through a cistern, the cohesion of the water will draw along with it that on either side, until the contents of the cistern will stand at a lower level than the stream itself; and Venturi, who discovered this fact, availed himself of it to drain a marsh near Modena, by turning through it a neighboring rapid stream. So, Magnus found a jet of about inch diameter directed upward through a spout of inch diameter in a reservoir to arrest in a degree the escape of liquid around it, and maintain the level within at some 10 inches above the orifice.-The force of impact between a liquid and a solid body moving in or against it, may arise from motion of either, or of both, and in the same or in opposite directions. If one only be moving, the resistance or impact is generally stated as the square of its velocity; if both, in opposite directions, as the square of the sum of the velocities; both in the same direction, as the square of the difference. A body having 1 square foot of surface, at right angles to its course, and impelled forward just below the surface of water with the velocity it would acquire in falling the first second (2 X 16, ft. a uniform velocity of 386 inches per second), will meet with a resistance equal to the weight of a column of water 193 inches in height and of 1 square foot base, i. e., about 1,003 lbs. This may be taken as the unit of resistance in such cases; but the resistance must be proportionally increased for greater depths, and calculated on the principle of resolution of forces, when the solid, like the bow of a ship, presents its surface obliquely, not at right angles to the liquid body to be displaced. At very great velocities, however, the resistance is found practically to be increased in much higher than the duplicate ratio above given; and owing to the mobility of water, long tapering bows, which give time for the gradual displacement of the liquid, do not experience, in proportion to that felt upon those more square-built, even the full theoretical resistance due to the amount and obliquity of surface they present.

HYDROMETER (Gr. idwp, water, and μerpov, measure), an instrument for determining the specific gravities of fluids, the principle of which has already been explained under AREOMETER. A variety of these instruments have been introduced in different countries. Baumé's is generally recognized in the United States, and on the continent of Europe is much used, especially for liquids heavier than water. It is made of two sorts, one for liquids lighter, and the other for those heavier than water, and of these there are varieties for special liquids. For acids or salts the instrument is graduated by sinking it first in pure water, and ballasting it so that the water line shall be near the top of the stem. This fixes the zero point. It is then floated in

a solution of 15 parts by weight of dry common salt in 85 parts of distilled water; and the point cut by the water line is marked 15°. From these the whole stem is divided into degrees, which should reach to 66° for sulphuric acid. The greater the degree indicated in its use, the more dense is the fluid. For spirituous liquors the zero point upon the stem is determined from a solution of 10 parts of dry salt in 90 of water, in which nearly the whole of the stem should be out of the fluid. The 10° point is given by pure water, and the degrees are from these marked upward, even to 70° for sulphuric ether; the larger the degree indicated the lighter is the fluid. Instruments for special uses are made from these, but with short range adapted for their particular service. The specific gravity of a liquid is ascertained from the indication of the hydrometer of the first class by the formula, g=152; and of the 2d class, by gaid being the degree given by the scale. But tables tier's hydrometer is also much used in commerce, are prepared for convenient. reference.-Carespecially by the French. It is made by adopting the 22° point of Baumé, and dividing the 16 degrees on each side of this into 15 equal degrees. The degrees of Cartier (C.) are converted into those of Baumé (B.), and reciprocally, specific gravity, g, corresponding to C. degrees by the formula, 16C=15B+22; whence the

=

136.8

126.1+ C.

146

The hydrometers of Baumé and Cartier are based on a temperature of 12.5° cent. (54.5° F.); and when used for a liquid at a different temperature, allowance is to be made, as given in the tables.-In Great Britain, Twaddell's hydrometer is in general use for dense liquids. It is so graduated that the specific gravity may be deduced by multiplying the degree indicated by 5 and adding 1000, water being reckoned 1000; thus 12° Twaddell indicates a specific gravity of 1060 or 1.06.

HYDROPATHY (Gr. vdwp, water, and rabos, affection or disease), a system of treatment of diseases mainly or exclusively by the use of water and of the known hygienic agencies. Hygienic management in some form, as a resort to exercise, or, in diseases induced by luxurious living, to abstemiousness, dates from the earliest conception of a healing art; and it has kept pace with the growth of physiological science, until within the present century the laws and claims of hygiene have become appreciated as never before. The physicians of very early times seem also to have employed water as a remedy in certain febrile, inflammatory, and surgical maladies; a usage recommended, among other early medical writers, by Hippocrates, Galen, and Avicenna. In the 18th century Sir John Floyer and Dr. Baynard, in England, resorted to bathing almost exclusively in chronic diseases; as did F. Hoffmann and Hahn on the continent. Dr. James Currie in 1797 published highly favorable reports of the effects of water, chiefly by affusion, in many diseases. But the distinctive "water cure," or hydropathy, owes

its origin to the fertility of invention of a Silesian peasant, Vincent Priessnitz. Having at the age of 13 sprained his wrist, young Priessnitz intuitively applied it to the pump; and afterward, to continue the relief thus obtained, he bound upon it an Umschlag, or wet bandage. Rewetting this as it became dry, he reduced the inflammation, but excited a rash on the surface of the part. Soon after, having crushed his thumb, and again applying the bandage, the pain once more subsided, but the rash reappeared. He inferred that the rash indicated an impure blood; and this conclusion was strengthened by the result of experiments which he was induced to try upon injuries and ulcers in the case of some of his neighbors, since the rash in some instances appeared after the treatment, and in others did not. Thus he was led to frame for himself a humoral pathology of all diseases, and a doctrine of the elimination of morbific matters by "crisis." According to this view, the cure of disease is to be effected by favoring the activity of those organs through which the purification of the system is carried on, and, through a regulated and pure dietary and correct regimen, preventing further morbid accumulations. In his 19th year, being run over by a cart, Priessnitz had some ribs broken and received severe bruises; on learning that the physicians pronounced his case hopeless, he tore off their bandages, and recovered under the renewed application of the Umschlag, and replaced his ribs by inflating the lungs while pressing the abdomen against a window sill. This incident confirmed the idea and inaugurated the practice of the water cure. In the new practice, its author discovered in rapid succession the means of securing either cooling, heating, or soothing effects by compresses; then, the sponge bath, the wet-sheet packing, the sitz, foot, arm, and other partial baths, the douche, the stream bath, the dripping sheet, the plunge, the tepid shallow bath, dry-blanket packing, &c. The pail douche of Dr. E. Johnson is one of the very few additions since made to this list of measures. Unquestionably, Priessnitz's earlier treatment, especially after the opening in 1829 of the famous Gräfenberg cure, was too incessant and severe, and often borne only through the vital tenacity, whatever their maladies, of the class of invalids with whom he had to deal. Along with this was introduced a rigorous, but in some respects mistaken hygiene, including the very free use of a plain and peculiar diet, much walking in the open air, and the disuse of flannel undergarments and of soft beds. The water appliances have since been rendered more mild, and in the United States necessarily so. The number of instances, however, of decided restoration to health among the invalids (many of them afflicted with the worst forms and complications of disease, and abandoned to incurability by the prevalent medication of the day) who flocked from all parts of Europe and of the United States to the Gräfenberg cure, sufficiently explains the rapid VOL. IX.-27

spread of the new system. This was first distinctly brought to the notice of the English public about the year 1840, by a book put forth by a former patient of Priessnitz, Capt. Claridge, and entitled "Hydropathy, or the Cold Water Cure." In Germany, under Francke, Weiss, Munde, and others, the enthusiastic treatise of the first of whom did much to spread the system, several new establishments had already sprung up. On March 17, 1842, the hydropathic society was formed in London, for the purpose, among others, of collecting information in regard to Priessnitz and the authenticity of the reported cures. Drs. Wilson, Johnson, and Gully were first to embrace the practice, the first two early lecturing before the new society, and all soon establishing institutions of their own. The writings of Drs. Gully and Johnson contributed much to spread the system in England, and at a later day they were ably seconded by Bulwer's "Confessions of a Water Patient," detailing incidents of his restoration to health at the Malvern establishment. The earliest popular information concerning water treatment in the United States was through a letter published about 1843, from H. C. Wright, himself at the time a patient under Priessnitz; and this was soon followed by the earnest statements and appeals, through a like channel, of J. H. Gray of Boston and A. J. Colvin of Albany. Drs. Schiefferdecker, Wesselhoeft, and Shew seem to have been the first to enter upon the new practice in the United States; while the first establishment appears to have been that opened in 1844 at No. 63 Barclay st., New York. Of this, David Cambell, also the originator of the existing "Water-Cure Journal," was proprietor, and Joel Shew physician. Early in May, 1845, was opened the establishment at New Lebanon Springs, N. Y., under Dr. Shew, and the more widely celebrated one at Brattleborough, Vt., under Dr. Wesselhoeft. The latter, having explored the country from Florida to Maine, selected Brattleborough on account of the superior purity of the water of a spring there. Probably there are at the present time, scattered throughout this country and Europe, more than 200 establishments of this kind. Several works relating to hydropathy, and of interest in a practical point of view, have been issued in the United States, especially by Drs. Shew and Trall; but in this respect the practice experiences the want, incident to the infancy of almost all systems, of a comprehensive and philosophical exposition of its principles and their application. A school in the city of New York teaching the doctrines of this system, perhaps the only one now in existence, received a charter from the state legislature in 1857.

HYDROPHOBIA (Gr. vdwp, water, and poßeopa, to fear; Lat. rabies canina, canine madness). This appalling disease has been long known. Homer speaks of madness occurring in dogs. Aristotle denies that man is ever affected by it, but soon after it was described as occurring in man; and from that time to this

notices of it abound among medical authors. The disease is propagated by the dog, the wolf, and the jackal; the cat likewise is capable of communicating it; and Youatt gives an instance in which a groom, whose hand was scratched by the tooth of a horse laboring under it, became affected with hydrophobia. Man and most if not all of the domestic animals are capable of contracting the disease, but whether they are capable of communicating it is still undecided. For man to contract it, the saliva of the mad dog must be applied either to a mucous membrane or to an abraded surface; the mere contact of the poison with the unbroken skin will not, except under very rare circumstances, produce the disease. The period that elapses between the reception of the injury and the outbreak of the disease varies exceedingly; the greater number of cases occur between the 30th and the 59th day; some have occurred earlier, and a few as late as 6, 7, 9, and even 19 months after the reception of the wound; in the one or two instances on record in which a still longer period has elapsed, the patient has probably been reinoculated with the virus. The first symptom which attracts the attention of the patient is pain or some uneasy sensation in the cicatrix of the original wound, which extends toward the trunk. The wound itself sometimes becomes red, livid, or inflamed, or it may open afresh, discharging a peculiar ichor. This is the period of recrudescence, and though not always noticed occurs probably in every case. Commonly within 2 or 3 days after this stage of recrudescence, rarely later than 6, during which period the patient remains ill and uncomfortable, the 3d and final stage of the complaint commences. A feeling of painful stiffness occurs about the back of the neck, extending to the base of the jaw and root of the tongue; the breathing is hurried, and sighing is frequent; the patient finds himself unable to swallow fluids, every attempt to do so bringing on a fit of the most distressing sobbing and choking; during this paroxysm the muscles of the mouth and pharynx are seen to be spasmodically contracted. The patient is tormented with thirst, and the paroxysms are often excited by the sight or sound of water; even a polished surface or a current of air is sometimes sufficient to bring them on. The mind seems excited; the patient appears alarmed, is anxious, irritable, and suspicious; often a degree of delirium or mania supervenes. He is annoyed by the secretion of a thick viscid saliva, and tries to free himself from it by blowing and spitting. Vomiting of a yellow or greenish matter is often present, sighing is frequent and peculiar, and the pulse rapidly becomes weak and frequent. Death commonly takes place on the 2d or 3d day, though it has happened within 24 hours, and has been delayed to the 9th day. In some few cases, toward the close of the disease, a remission of the symptoms takes place, when the patient is able to eat and drink, and appears composed; in this condition he may sink into a sleep which is but the precursor of death, or

he expires in a sudden and general convulsion. Examination after death throws no light on the phenomena of hydrophobia. There are no appearances found peculiar to it. Evidences of inflammation of the oesophagus, pharynx, and larynx are commonly present, but they are results rather than causes of the disease.-It has been maintained, and is still maintained by some authorities, that there is no such disease as hydrophobia, and that the phenomena are caused by fear acting on a sensitive organization. The uniformity of the symptoms, the effect of the poison upon other animals, and the occurrence of the disease in infants and idiots, refute this hypothesis. Hysteria sometimes imitates hydrophobia, as it does every other disease, and apprehension has reproduced some of its symptoms, but these cases can readily be discriminated. Some well authenticated cases are on record where the disease has occurred and proved fatal, and yet in which the patient had not been bitten by a rabid animal, so that a hydrophobia of spontaneous origin has been admitted by most authorities; but in these rare cases may not the origin of the inoculation have escaped notice? Some singular instances are related by Mr. Youatt of the mode in which the poison may be propagated. A man was observed attempting to untie a knot in a cord with his teeth; two months after he perished of hydrophobia. The cord had been used to tie up a mad dog. A woman had her dress torn by a mad dog, and in mending it she pressed the seam with her teeth and was inoculated. Of a number of persons bitten by a rabid dog, how many will be attacked with hydrophobia? J. Hunter says that in one instance of 21 persons bitten only one was attacked. Dr. Hamilton thinks the proportion is 1 in 25. In a case that occurred near Senlis in France, where by the order of the government the most accurate inquiries were made, out of 15 persons bitten 5 were attacked. The bite of the rabid wolf seems more fatal than that of the dog; thus out of 114 persons bitten by rabid wolves 67 died.-When hydrophobia is characteristically developed, the patient dies; there is no authenticated case of cure on record. By the use of chloroform and narcotics we may relieve the agony of the patient, but that is all. As a prophylactic, the only sure means is the complete excision of the part bitten. To accomplish this, Abernethy recommends that a bit of wood sharpened to a likeness of the dog's tooth be thrust into the part and then cut out without the knife's coming in contact with the wood. Where this will not be permitted, caustics may be liberally employed, and of these the best are perhaps nitric acid and strong liquid ammonia. Mr. Yonatt uses lunar caustic with success.

HYDROSTATICS. See HYDROMECHANICS. HYDROSULPHURIC ACID, SULFHYDRIO ACID, or SULPHURETTED HYDROGEN, a gaseous compound first examined by Scheele in 1777; symbol, SH; chemical equivalent 17. It consists of two volumes of hydrogen and one of sulphur