HYDROSTATICS. 385 HYDROTHERAPY. the comparison of the densities of two liquids which do not mix. Flotation. If a body less dense than a liquid is immersed in the liciuid and allowed to come to equilibrium, it will rise to the surface and 'lloat' with only part of its volume below the surface. By Archimedes's principle the upward force equals the weight of the liquid displaced; and, since the body docs not move in a vertical direc- tion, this force must equal the weight of the body itself. Therefore, a floating body displaces a volume of the liquid of weight equal to its own. The upward force acts through the centre of gravity of the space formerly occupied by the liquid displaced, while the dowTiward force acts through the centre of gi'avity of the floating body. If the body is in equilibrium these two forces must have the same line of action, or the two centres of gravity must lie in the same vertical line. In most eases this equilibrium is stable, Init it may be unstable, e.g. an oblong block .set floating with its long direction vertical. The test of stability is to give the floating body a small displacement, i.e. tip it slightly, and to see if the forces acting on it tend to restore it to its previous position or to make it tip still further. See JIistacentre. Capillaritt. Where the freer surface of the liquid meets the wall it is not horizontal ; and, when a tube of fine bore dips into a liquid, the free surface inside the tube is at a different level from that outside. These variations are said to be due to capillary action, and they may all be shown to be due to the fact that a liquid surface contracts so as to have the smallest area com- patible with existing conditions. The principle of Arcliimedcs. discussed above, is attributed correctly to the philosopher of Syracuse ; and many facts in regard to liquids were known to him and other scientists of an- tiquity. Galileo first stated the law of connect- ing tubes, and Pascal was the first to recognize the fact that the reaction of the walls pro- duced a pressxire which was the same at all points throughout the fluid. The ordinary state- ments and proofs of hydrostatics are given to- day exactly as they were by Stevinus (1548- lfi20). For further information, consult Green- hill. nijdrostatic.<! (London, 1894). HY'DROSTTLPHTJ'RIC ACID. See SuL- rmuETEi) Hydrocen. HY'DROTHER'APY, or HYDROP'ATHY (fromGk. i/itup, /i,i/<?rjf, water + BtpoTrcla, thera- peia, cure, from ffepaTreieiv, therapeuein, to cure, from Bepairuv, ihrrapOn, attendant). Water treatment. ' The efficacy of water in the cure of numerous forms of disease has long been recog- nized. Water was largely employed by Hippoc- rates in the treatment of many kinds of disease. Horace speaks of Antonius JIusa. the hydropathic physician of the Emperor Augustus (Epist. i. 1.5). Both Celsus and Galen speak favorably in their writings of the use of water in the cure of disease, regarding it as of high vahie in the treat- ment of acute complaints, particularly of fevers. Throughout the Middle Ages, likewise, many physicians, including Aetius, Paulus .^gineta, and Paracelsus, were advocates of the remedial virtues of water; all of them, however, having faith in its uses in the treatment rather "f acute than of chronic disorders. In 172.3 Niccold Lan- zani, a Neapolitan physician, published a learned treatise on the subject. In England, about the beginning of the eighteenth century. Sir John rioyer and Dr. Baynard made large use of water. Their conjoint work, denominated I'sy- chrolousia, or the "History of Cold Bathing, both Ancient 'and Modern," is replete with quaint learning and practical shrewdness and sagacity. But the most al)le and scientific among the older treatises that have a|)peared in England on the subject of the water treatment is the work of Dr. Currie, published in 1797, entitled Medical Reports on the Effects of Water, Cold and Warm, etc. In this work Currie recommends the cold affusion in typhus and other fevers, and gives practical directions in regard to the cases and the times when it may be used with advantage, although he appears to have limited his use of water to acute ailments exclusively. We have thus seen that up to the beginning of the nineteenth century, by some of those who employed it as a curative agent, water was used in the treatment of acute, and by others of chronic, diseases; by some as an internal agent alone; by others as an external application in the various forms of the bath ; but never in all the manners combined. This combination was first effected by the original genius of Vincent Priessnitz, a Silesian farmer, with whom began a new era for the water cure. It was owing, we are told, to his successful treatment of more than one bodily injury which he had sustained in his own person that, about the year 1820. Priessnitz became so convinced of the curative powers of water as to employ it medically in the cure of others. Beginning with the external application of water for trifling diseases among the poor of his neighborhood, he gradually undertook an ex- tended range of eases, and multiplied the modes of administration, introducing the wet compress, the douche bath, partial baths of all kinds, the sweating process, the wet sheet, together with copious drinking of pure water. In addition to water in all these forms, he insisted on the value of exercise, diet, fresh air, and mental repose in the cure of disease, thus practically calling to his aid the entire resources of hygiene, and estab- lishing by a simple, yet thoroughly original com- bination, nothing less than a new system of medical treatment. As to the success which at- tended Priessnitz's practice, it is an historical fact that of 7.500 patients who had gone to Grjl- fenberg for advice and treatment up to the year 1841. or within the space of about twenty years, there had been only 39 deaths. It is to be re- gretted, however, that the founder of the new system was not himself an educated physician, so that he could have understood better the phi- losophy of his own practice and explained it more correctly. He would not have called his system the 'water cure.' a name scientifically one- sided and incomplete, and therefore misleading. The undoubted merits of hydropathy at length called to its defense many men of standing in the profession, who, allowing for some of its early extravagances, explained it scientifically, and from their advocacy has sprung up a school of hydropathic physicians. Dr. Winternitz, of Ger- many, in 188.'? laid down the scientific principles of modern hydrotherapy. The fundamental prin- ciples of hydrotherapy are very simple. The art of applying these principles requires much teach- ing, but any one with even inoderate sense and intellect can grasp the essential features. Water
