PRIME-MOVERS AND DIRECT-ACTORS. 505
driving link to replace those which have left the machine. It causes the water to move as it were in a circle, always raising it again after it has passed downwards through the machine.
The hydraulic ram mentioned in 129 no longer presents any difficulties to us. The water in it is kinematically chained with the other parts, the whole forming, in fact, a ratchet-train. So far as the descending water driven by gravity is concerned this train is reversed, but in respect to the portion of the same water which is raised it is direct. The liquidity of the pressure organ allows it to be thus separated into two streams. It is quite indifferent to us, and in no way affects our definition, that the water here both drives, is driven, and communicates motion. In every case we see that the driving body, the driver or motor, forms itself a link in the kinematic chain, instead of being, as in the old conception of the machine, entirely external to it.
134. Prime-movers and Direct-actors.
We have now reached a position which enables us to give an answer to the question formerly raised ( 129), whether the steam-engine, the water-wheel and turbine, the lathe and planing- machine, the loom, the crane, and so on, could .or could not each by itself be considered a complete machine.
In regard to the three first we can at once say that they are complete machines ; they are moreover place-changing machines, giving to certain of their parts, by suitable kinematic chaining, a determinate motion which may be utilized for any desired end. A steam-engine, for example, may be employed to drive machinery of the most different kinds without in the least altering its own mode of action. The various uses to which portable engines are put gives us a familiar illustration of the way in which advantage is taken of this fact in practice. One prime-mover may always be substituted for another without any alteration in the machines driven by it, if only the effort applied to the shaft and the speed at which it is driven remain unaltered. In other words, the machine fulfils its end in these cases if it give to one link of the chain a uniform rotation, or cause its points to undergo changes
