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 HYDROMECHANICS 115 t'ion a very great degree of mutilation, each fragment into which they may be divided be- ing capable, according to Trembley, of be- coming a complete individual. Reproduction is either non-sexual, by gemmation in summer, or sexual, by ova and sperm cells in autumn ; Hydra. the buds develop a mouth and tentacles at the free end, and are soon detached, each in its turn producing similar buds ; both ova and sperm cells are produced in the same individu- al, coming in contact in the water ; the em- bryo is at first ciliated and free swimming, afterward becoming fixed, losing the cilia, and developing a mouth and tentacles. HYDROMECHANICS, that branch of natural philosophy which treats of the mechanics of liquids, or of their laws of equilibrium and of motion. It includes the consideration of those molecular properties of liquids which affect their mechanical applications, such as fluidity and slight compressibility. The science which is here termed hydromechanics has been some- times treated under the title of hydrodynam- ics, this being made to include hydrostatics and hydraulics, which is the nomenclature adopted by Sir David Brewster ; while others treat of hydrodynamics and hydrostatics as two independent subjects, hydraulics being embraced by hydrodynamics ; but the title hydromechanics which was adopted in the first edition of this Cyclopaedia seems to be the most comprehensive and exact, and will be re- tained. Hydromechanics is comparatively a modern science, having received its greatest development in the 16th, 17th, and 18th cen- turies. The ancient mathematicians and hy- draulic engineers, who constructed the aque- ducts of Egypt and Assyria, must have been acquainted with many of the more obvious principles of hydraulics and hydrostatics ; and at the time of the construction of the Roman aqueducts hydromechanics may be considered as having become entitled to be called a sci- ence ; but the more purely mathematical prin- ciples by which its laws can be well under- stood were not discovered till centuries after. Some of the general principles which lie at the foundation of the science, and are suscep- tible of analytical and experimental demonstra- tion, were first given by Archimedes in the latter part of the 3d century B. C. ; and it is to him that we owe the demonstration of the fundamental principle of the equilibrium of liquids, that each particle in a liquid at rest receives equal pressure in every direction, and also that a solid immersed in a liquid loses an amount of weight equal to that of the water displaced, from which he deduced the method of obtaining the specific gravity of bodies. We also owe to him the method of raising water by means of the screw known by his name. Other advances in the construction of hydrau- lic machinery were made about the same time in the Greek school at Alexandria by Ctesibius and Hero, who invented the syphon and forc- ing pump, and also the fountain known as Hero's; hut their limited knowledge of pneu- matics, and the imperfection in the machinery of those times, prevented them from bringing the force pump to anything like its present degree of efficiency. The first attempt at a scientific investigation of the motions of liquids was made by the consul Frontinus, who was inspector of the public fountains at Rome un- der the reigns of Nerva and Trajan, and whose book De Aquaductibus Urbis Romm Commen- tarius, describing the nine great aqueducts of Rome, to which he afterward added five, con- tains all the knowledge of hydromechanics pos- sessed by the ancients. From the statement of Pliny that water will rise to a level with its source, and that it should be elevated in leaden pipes, it appears that this metal was used by the ancient Romans for small conduits. Frontinus was the last of the ancients who paid much at- tention to the subject, the next investigator of importance being Stevinus, born about 1550, who was engineer of dikes for the government of Holland. He published a work in Dutch in 1586 on the " Principles of Statics and Hydro- statics," in which he restates the principle of Archimedes, and deduces from it the " hydro- static paradox," that the pressure of a liquid on the bottom of a vessel may be much great- er than its weight. By a method approaching the infinitesimal calculus, he found the pres- sure on the oblique bottom of a vessel ; and AVhewell remarks that his treatment of the subject embraces most of the elementary sci- ence of hydrostatics of the present day. Ga- lileo, in his " Discourse on Floating Bodies " (1612), shows a clear knowledge of the fun- damental laws of the science ; and it is to his discovery of the uniform acceleration in fall- ing bodies that we owe one of the chief foun- dations of hydromechanics. This law was afterward more fully applied by Torricelli in his celebrated theorem that the velocities of liquid jets are proportional to the square roots of the depths at which they issue below the surface, which he published at the end of his treatise De Motu Graviwm naturaliter accel-