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 fix in position one linkof the chain relatively to the portion of surrounding space assumed to be stationary. The relative motions of the links then become absolute. A closed kinematic chain, of which one link is thus made stationary, is called a mechanism.
Fig. 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:—
| 1. | b — c | d — e | f — g | h — a |
| 2. | b — c | d — e | f — g | h — a |
| 3. | b — c | d — e | f — g | h — a |
| 4. | b — c | d — e | f — g | h — a |
In general, therefore, a constrained closed kinematic chain can be formed into a mechanism in as many ways as it has links.8
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, for example, can be clamped, so that kinematically it may form one piece with . 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
