Page:The New International Encyclopædia 1st ed. v. 10.djvu/434

HYDROGRAPHY. 878 HYDROMETER. Office through its surveys, and from other sources of iiiformatioii. is quite as important as the work of colleotiiif; such facts. In this way only is it l)ossible to make the colk'cteU facts of value. So far as the mariner is concerned the most im- j)ortant work of the llydrographic Office is the charts of navigable waters which it issues. These charts are made up by compiling the data col- lected as previously described, and then construct- ing maps or charts, showing the configuration of the coastline and the ocean bottom, the depths of water, the location of channels, rocks, reefs, buoys, lighthouses, etc. As changes occur in these particulars tlicy are recorded, and new charts made showing the changed conditions. In- formation which is not suitable for representation on charts is published in book fonn, or in peri- odicals called Xoliccs to Mariners. When it is stated that the llydrograpliic Odice of each na- tion publishes not only its own charts, reports, and notices, but reproduces a considerable por- tion of the similar information published by other nations, it will be seen that the task of recording and making public hydrographic in- formation is one of magnitude. See Hydbogra- PHic Office. HY'DROID (from Gk. vApouiii^, hydrocidCs, like water. Irora iJup, hi/dOr, water -|- fiVof, eidos, form). One of a class of cadenterate ani- mals, notable for their delicacy and beauty, and receiving their name from their structural re- semblance to Hydra (q.v.). The name is now generally restricted to the polyp fonns of the }lydrom"edus.Te. (See HYnRozo..) Thej- exist in compound colonies, one kind having the office of feeding the community, another of protecting it, and another of reproduction. The feeding hydroids are usually fixed, or attached to some object, and proceed from eggs of the reproduc- tive hydroids, or medusir. the latter in turn grow- ing from buds produced by the former. The niedusie sometimes remain attached to the stem, or become free-swimming. The body of the nu- tritive hydroid is usually supported by a stem of variable length, but may rest immediately upon the bottom. From one individual buds ai>pear and produce branching colonies of hundreds or tliousands, often l.aving a height of fifteen or twenty inches, and the giant hydroid of .Japan is more than three feet high. The reproductive hy- droids are sometimes developed into perfect meduste before leaving the parent stem, but they usually break away before attaining their per- fect state. Some buds never become much de- veloped, and are called gonophores. These usual- ly remain attached, but attain sexuality and re- productive power. Most hydroids are covered with a chitinous envelope, which is continuous over the branching stem of the entire colony. Vnit some species are naked, and soft. In tubularian hydroids, the chitinous envelope, when present, simply incloses the stem and branches, but is not expanded around the individual polvps (h;/- dranihs) in the form of a cup. In campanula- rian hydroids, however, each hydranth is sur- rounded by a cup, a continuation of the chiti- nous envelope, into which it can withdraw itself. Hydroids abound in the oeean in various parts of the world, notably the northwest coast of Amer- ica, the Caribbean Sea. and around Auslr.ilia. Their colors are usually not brilliant, brown, fleshcnlnr. and white tieing the most common. A familiar and beautiful example is the Portu- guese manof-war (q.v.). Consult Agassiz, Xorlh Ameriaiii Acalcphw (Museum of Comparative Zoology, Cambridge, 1875). Sec SiruoNOi'noRA, Al.TER.NATioN OF Ge.nerations, and the accom- panying illustration. HY'DROMAN CY. See Divination. HYDROM'ETEB (Gk. Idiw/iirpiov, hijdrome- Iriun, vessel for hydrostatic measurement, from viuf), hydor, water + /lir/mi', iiiclruii, measure), also known as Abeometek. An instrument used in determining the specific gravity of liquids, and in some instances of solid bodies. Specific gravity, for determining which the hydrometer is used, is the ratio between the weight of a given quantity of a substance and that of an equal volume of water at its tempera- ture of maxiuuim density, 4° Ceutigrade, so it follows that the specific gravity of two sub- stances must be directly proportional to their weights when the volumes are the .same, or in- versely as their volume-s when the weights are equal. The hydrometer, which is a hollow in- strument of glass or metal designed to float up- right in a liquid, makes use of the principle of Archimedes that the weight of the volume of liquid displaced by a body is equal to the weight of the body itself. In its simplest form a hydrom- eter might consist of a graduated scale floating vertically in a liquid, and on which the level of the surface of the liquid it disjjlaces could be measurt'd. For example, an ordinary ruler if allowed to float upright in water would sink un- til a certain division on the scale is reached. This reading multiplied by the area of the eross- Eection of the ruler will give the volume of a mass of water equal to that displaced by the ruler, and equivalent to the weight of the rulei itself. Immersing the ruler in a licpiid of dif- ferent specific gravity, it will sink until the sur- face touches some other division on the scale, and as before the volume of a ma.ss equivalent to the weight of the ruler will be obtained. A ratio made with these two quantities representing the volumes of the two liquids would give the spe- cific gravity, but as the cross-section of the ruler is a constant quantity, it is only necessary to compare the two readings on the scale. The ordinary' hydrometer consists of a glass tube teniiinating in two bulbs, the lower of which is filled with mercury or shot in order to keep the instrument steady in an upright position when immersed in a liquid. Such a hydrometer is ex- tensively used in scientific and commercial work, as it can be graduated to furnish direct readings of the specific gravity of a liquid. The mark to which the instrument Avould sink when placed in water is usually marked 1.000. and the weight of water dis]>laced is equal to the total weight of the hydrometer. If the instrument be placed in a liquid that has a less specific gravity than water, alcohol for example, it will sink until the surface of the liquid touches some point higher ip on the stem, as in this ease it takes a greater volume of the lighter liquid to he equivalent to the weight of the hydrometer. The scale on which the readings are made i^ on a piece of paper contained inside of the stem, and the method of graduation forms the chief distinction between the difl'erent instruments. In the in- strument of Gay-Lussac. which i,s known as a volumeter, the water-point was marked 100, and the division was carried along the stem both