Page:The Kinematics of Machinery.djvu/69

 In itself a closed chain does not postulate any definite absolute motion. In order to obtain this a similar method must be adopted to the one employed above with pairs of elements,—namely, to hold fast or. The relative motions of the links then become absolute.

. 10.

The above chain can be made a mechanism in four different ways, as shown in the following table, in which the stationary link is underlined in each case:—

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In order that a link may be made stationary it must be provided with suitably formed fastenings or carriers.

To make the demonstration complete, let us suppose that we employ a sufficiently rigid pedestal, such as that shown in Fig. 11, as a support to which one link of our chain, $$a h$$ for example, can be clamped, so that kinematically it may form one piece with $$a h$$. The motion which can now take place in the chain is indicated by the dotted lines, and will be at once recognised as that of the beam and crank of a steam-engine.

The form of the pedestal or equivalent body is of course, so far as the motion is concerned, indifferent. Yet it will be noticed at