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HYDRA

903

HYDROMETER

suburb, has good schools and a free library, and is served by the N. Y. N. H. and H. Railroad. It was incorporated in 1868. Population i5»5°7-

Hy'dra, a small fresh-water animal of low grade of organization. It has a tubular body, about the size of a knitting needle and from one quarter to five eighths of an inch long. This is capable of being extended and contracted into almost a globular form. One end of the body is attached, and the other is developed into a circular disc surrounded by a circlet of six or eight tentacles. In the middle of the disc is a mouth which leads into a general cavity or stomach. The body, therefore, is a hollow tube. It is composed of two cell-layers, an outer and inner layer (ectoderm and endoderm). A thin supporting layer is formed between the two, which foreshadows the middle layer (meso-dernt) found in connection with the others in all higher animals. The hydra has two cell-layers, but the bodies of all higher animals have three cell-layers. (See DEVELOPMENT,) Hydras live singly, and propagate both by_ budding and by eggs. The budding is very interesting. When they are well-fed, a bud will start from the parent forms which soon develop into tubular bodies. Later, discs and tentacles are added. The buds are usually set free after the development of a mouth, but sometimes a second bud (or more) will start and reach the feeding condition before any are set free. In many marine animals (hydrozoa) a similar process of budding occurs, and the new forms produced all remain together permanently. The entire group grows, and budding continues until branching colonies of considerable extent are produced. See CORAL and HYDROZOA.

Hydrau'lic En'gines or Mo'tors, are often used where water can be got under high pressure,, They do not differ essentially from steam engines (which see). The water acts by difference of pressure — that is, it is let in at a high pressure at the beginning of the stroke, and let ouc at a low pressure at the end of the stroke, thus giving a back-and-forth motion to the piston. The speed of the piston has to be kept low to avoid hurtful shocks in suddenly bringing the column of water to rest. Since hydraulic engines work under very much greater pressures than steam engines — usual pressure 700 pounds per square inch — they can be much smaller. A common form is the three-cylinder, single-acting engine. In each cylinder works a plunger; water is let in by valves behind the plungers, and forces them out. At the end of the out-stroke, the water pressure is cut off, and the exhaust valve opened, allowing the plunger to push the water out of the cylinder on the return stroke, and so on.

Hy'drau'lics, the science which treats of the flow of water. It is partly theoretical, based on the general laws of fluid motions

as developed in physics, but largely experimental. It has to do with the flow of water through orifices, pipes and canals and over weirs and dams and with the use of rivers and streams for power purposes. (See WATER-WHEELS.) The designing of canals, aqueducts and pipe-lines for drainage and public water-works is an important application of tke science of hydraulics.

Hy'drochlo'ric Acid. See ACID. Hy'drocyan'ic Acid. See ACID. Hy'drogen, one of the elements and the lightest gas known. It is colorless, odorless, and not poisonous. It burns with a non-luminous flame, which produces great heat and forms water. At a very low temperature it can be changed to a liquid and, finally, to a transparent solid. Liquid and solid hydrogen are only about one fourteenth as heavy as water, and in these conditions also hydrogen is the lightest known substance. Combined with oxygen, it forms one ninth part by weight of water, and it plays a most important part in the makeup of tissues of animals and plants. It also is in a large number of manufactured substances and products, as starch, sugar, vinegar, gutta-percha-alcohol, ether, benzine, aniline, indigo, mor, phia etc. In fact, it is a constituent of practically all organic substances. It is not found largely in a free state, but in some places it comes out of the earth with other gases in natural gas, as in the petroleum regions of Pennsylvania. It is usually prepared by the action of acids on metallic zinc or iron. As it is the lightest gas known, it is used as a standard to measure the densities of other gases. The air of the atmosphere, for example, is 14.5 as compared with hydrogen as i; moreover, as the weight of an atom of hydrogen is less than that of an atom of any other element, the atomic weight of hydrogen is taken as a measure, and those of other elements referred to it. Compounds containing hydrogen and one other element are products of decaying vegetable and animal matters, as marsh gas, ammonia and hydrogen sulphide.

Hydrom'eter, an instrument employed in science and commerce to determine the densities of bodies. It consists essentially of a slender floating body, heavily ballasted at the lower end. There are two principal forms. The simpler form is made of glass, the lower end being provided with a bulb partly filled with mercury as ballast, the upper end being a hollow graduated s t e m. This form is marked A in the accompanying figure. The depth to which the stem sinks evidently depends upon the density of the liquid in which it is

HYDROMETER.