all cases at least two bodies correspond in being reciprocally envelopes each of the other. A machine consists solely of bodies which thus correspond, pair-wise, reciprocally. These form the kinematic or mechanismal elements of the machine.
The shaft and the bearing, the screw and the nut, are examples of such pairs of elements. We see here that the kinematic elements of a machine are not employed singly, but always in pairs; or in other words, that the machine cannot so well be said to consist of elements as of pairs of elements. This particular manner of constitution forms a distinguishing characteristic of the machine.
Fig. 5.
Fig. 6.
If a kinematic pair of elements be given, a definite motion can be obtained by means of them if one of the two be held fast or fixed in position,—that is, be brought to rest relatively to a given portion of space chosen with reference to the motion which is to be observed. The other element is then free to be moved, but only in the one particular way allowed by the constitution of the pair. Its motion relatively to its companion element is under these conditions the same as its absolute motion in the given portion of space. Thus, for example, in the pair of elements shown in Fig. 5,—a screw and nut,—if the former be fitted with a suitable foot or its equivalent, so as to prevent its motion, then each point in the nut, if it be moved, will describe a helix of determinate magnitude.
