Hydromechanics
a term sometimes used to include hydrodynamics and hydrostatics, as also hydraulics, or the department of engineering which deals with the application of liquids in motion to machinery. Hydromechanics is sometimes limited to the latter department alone. On the other hand, hydraulics is sometimes made to cover hydrodynamics.
ZO- HYDROIDA (hi-drō'ē-dä), in ology, a typical sub-class of Hydrozoa.
They have an alimentary region or polypite provided with an adherent disk or hydrorhiza and prehensile tentacles.
Unlike hydra, the type of the sub-class, most of the hydroida live in societies, each of which constitutes a polypidom so like a seaweed that it is often collected as such. But while the apertures on an ordinary seaweed are only minute pores, the Hydroida have little cup-like HYDROMETER cells along the stem and at the extremities of the branches, in which the individual 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 organism. The sub-class is divided into five orders, Hydrida, Corynida, Sertularida, Campanularida, and Thecomeduse. The Polyzoa or Bryozoa, once ranked with the sub-class, now figure as a distinct class, arranged under the subkingdom 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 elongated stem, graduated to indicate the density of the liquid by the depth to which it sinks.