HYDBOMETER 98 HYDROSTATICS cells along the stem and at the extremi- ties of the branches, in which the indi- vidual polyp lives. The cells have a small hole at the bottom, and the stock is hollow, so as to enable the individual polyps to remain part of a compound or- ganism. The sub-class is divided into live orders, Hydrida, Coi'ynida, Sertula- rida, Campanularida, and ThecomedussB. The Polyzoa or Bryozoa, once ranked with the sub-class, now figure as a dis- tinct class, arranged under the sub- kingdom Molluscoida. See Hydrozoon. HYDROMETER (hi-drom'e-ter) , an instrument for determining the relative densities of liquids. Distilled water is usually referred to as the standard of comparison. It consists essentially of a bulb or float weighted at bottom so as to float upright, and having an elon- t,'ated stem, graduated to indicate the density of the liquid by the depth to which it sinks. HYDROKINETICS, that branch of hy- drodynamics which treats of the applica- tion of forces producing motion in fluids, having thus to do with the flow of liquids in inpes, its issue from orifices under certain pressures, etc. HYDROMANCY, a form of divination practiced by the ancients; it consisted in interpreting symbolically the colors and images seen in water. HYDROPATHY (hl-drop'a-the). See Hydrotherapy. HYDROPHOBIA, rabies, from bite of mad dog, more rarely cat, wolf, or fox; a contagious disease, the result of a specific poison. The great danger lies in the fact that a person bitten by a supposed mad dog imagines or simulates its symptoms, especially if nervous or hysterical; whereas only a few of those bitten by a mad dog take the disease. The average period of incubation is 40 days, but it varies from 15 days to two years. Immediate cauterization or ex- cision of the part is the only effective remedy. The Pasteur method of inocu- lation is as follows: A rabbit is inocu- lated with a fragment of the spinal cord of a mad dog. The animal is affected with hydrophobia in the space of about one fortnight. A portion of its spinal cord is employed to inoculate a second rabbit, which also contracts the disease, but more rapidly; the spinal cord of this second rabbit serves to inoculate a third, and so on. When the spinal cord of these animals, which have died of hydrophobia, is suspended in a perfectly dry tube its virulence diminishes by de- grees and at last disappears. A collec- tion of these spinal cords, some of them entirely stale and powerless, others more fresh and active, others, again, quite fresh and extremely active, are always kept in readiness. To render a dog in- susceptible to rabies he is first inoculated with the stale and powerless sjvecimens, then with fresher and more active ones, and lastly with the most powerful of all, when he becomes quite proof against the inoculation of rabies. Pasteur claimed that by the same process he could pro- duce in the human being a correspond- ing immunity against hydrophobia. The difficulties that surround the subject will be appreciated when it is remembered that (1) 60 per cent, of people bitten by mad dogs do not develop hydrophobia; (2) the incubation of the disease is some- times extremely long, cases having been known to be deferred till two years af- ter the bite. HYDROPHYLLACE^ (hl-dro-fil- as'e-i), a natural order of herbs and bushes containing about 80 known spe- cies, natives chiefly of the colder parts of America. None of them are of im- portance; but some of them are favorite ornaments of flower-borders, particularly different species of nemophila. HYDROSCOPE, an instrument for measuring the dampness or moisture of the air or of any other gas. HYDROSTATIC PRESS, a machine in which the pressure applied to a small piston is by means of water transmitted to a larger one; great pressure is thus produced, the increase being in the pro- portion of the excess of the sectional area of the one piston over the other. In ap- plication the different parts of a hydro- static press occupy every possible posi- tion with rea'ard to each other, and in many the force pump is worked by hand. Great force, with slow motion, is com- municated to a piston by means of wa- ter forced into the large cylinder in which it moves, by a forcing pump of small diameter, to which the power is applied, the principle involved being the same as in the hydrostatic bellows. HYDROSTATICS, a department of science which treats of the conditions of equilibrium in liquids. For a liquid to remain at rest in any vessel, first, its surface must be everywhere at right an- gles to the forces which act on the mole- cules of the liquid, and, second, every molecule of the mass of the liquid must be subject in every direction to equal and contrary pressure. If the same liquid be placed in several vessels freely communicating with each other, it will stand in them all at the same horizon-
