174 KINEMATICS OF MACHINERY.
tension-organs can be very readily united into pairs of elements with rigid bodies of various forms. Thus we may have a rounded bar (Fig. 124) over which they slide, force-closed in both direc- tions, a pulley (Fig. 125), the tension-organ moving upwards on one side of it, downwards on the other; a drum (Fig. 126) upon which it can be coiled, and so on. We find these pairs of ele- ments used and applied in the most various ways, in pulleys and cranes, in belt- trains, in rope-trains, in submerged rope towing, etc. The elements always reciprocally envelope each other, but although the envelopment passes sometimes into enclosure, they have not the latter as an essential characteristic ; they must be classed therefore as higher pairs of elements.
Directly opposed to tension-organs there stand others which possess molecular immovability only in reference to compressive forces, and which may therefore be called pressure-organs. To this class belong all fluids, liquid and gaseous, water, oil, steam,
��FIG. 127. Fro. 128.
air, etc. The force-closure applied to them must be such as con- tinually presses their molecules together. In order, however, that they may not alter their form by extension on either side, all the surfaces of the fluid body besides those normal to the direction of motion must be pressed together with the same force. This is done by the help of latent forces, that is, by enclosing the fluid in vessels of suitable form and resistance. This occurs, for example, in steam- or water-pipes (Fig. 127), and in the cylinders of pumps, or of steam- or blowing-engines (Fig. 128), and so on. It hardly needs to be pointed out how extremely important a part pressure-organs of this kind play in the construction of machines.
Another class, differing but little from the one just described, has been formed of late years from some of the tension-organs, by enclosing them in suitably shaped vessels, and having thus rendered side way motion impossible, using them as pressure-organs. The
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