498 KINEMATICS OF MACHINERY.
kinernatically paired or chained with the driving parts. In general this occurs in such a way that it is difficult to point out the receptor in any single piece.
The motors of which we have spoken, water, wind, steam and other gases, are all pressure-organs. Looking generally at the prime movers driven by them so as to classify their leading characteristics, one important fact appears which we must not leave unmentioned. We find two different methods of utilizing the driving energy of the motor, corresponding to two distinct classes of prime-movers. One of these classes, which includes all the machines working with pistons, we have already seen to be ratchet gear, or more fully, reversed ratchet gear. The other class, to which water-wheels, turbines, wind-mills, etc., belong, are characterised by the continuous, or very nearly continuous, motion of their working fluid. This acts no longer periodically or reciprocally, but enters continually at one side and passes away at the other. In water-wheels its action might be imitated by a rack ( 61), in the wind-mill and in some turbines by a screw, in others by a rope passing down and up round a sheave, etc. The difference between the two classes may be expressed by using names which indicate the principal characteristics of their motion, calling the second running-gear as distinguished from the first, the ra'tchet-gear. All prime-movers driven by pressure- organs are either ratchet-gear or running-gear.
If we glance again at the chamber-crank and chamber-wheel trains which are described in former chapters, we see that they belong partly to the one and partly to the other class. The chamber- crank machines, both pumps and engines, are partly ratchet-gear and partly running-gear, some of them, indeed, taking a kind of intermediate position between the two classes; the chamber-wheel machines are essentially running-gear. For the purposes of many machines running trains are extremely con- venient, because their rotary motion can be so easily and directly utilized. The attempt to design rotary engines and pumps is essentially an endeavour to substitute running-gear for ratchet-gear in machines working with pressure-organs. 56
The common clock may serve as an illustration of a machine moved by the action of a weight. At first sight it appears scarcely doubtful that the cord or chain from which the weight
