FLECTION AL KINEMATIC ELEMENTS. 175
flat-link chain of the Neustadt cranes (Fig. 129) is enclosed in a tube, so that it can be pressed forward ; the thin brake band of steel has been made capable of resisting pressure at its free ends by bedding it in a hollow cylinder (Fig 130), its particles then acting like the stones of an arch. Wire-rope has also been used in a somewhat similar way,
The pressure-organs form closed pairs of elements like the lower pairs ; on account, however, of their molecular moveability, they must be classed with the higher pairs.
If we compare the two classes of pairs of elements to which the consideration of force-closure has led us, we see that they are closely related. They both have the peculiarity that* they can be used in one way only ; with a closing force, that is, of one parti- cular kind or direction, the tension-organ only with tension, the pressure-organ only with thrust. If the rope in Fig. 126 be pushed
���FIG. 129,
FIG. 130.
upwards, it does not set the drum in motion, nor can the piston (Fig. 128) be moved by withdrawing the fluid from behind it. The pair are closed on one side only; they are mono-kinetic, a peculiarity which we shall find further on to belong to other pairs also. They owe this to their molecular yieldingness, or want of fixedness in all except a single or a small number of directions. This quality is what is known in pressure-organs as fluidity, in tension-organs as flexibility or pliability. As a common designation for both, we may use the word Sectional, and therefore call both tension- and pressure-organs, when employed as kinematic elements, flectional elements.
The two sets of organs stand opposed to each other as positive and negative, a relation directly indicated also by the nature of their closing forces. The pipe filled with water, Fig. 127, stands opposite to the rope in Fig. 124 ; the cylinder having a piston moved
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