Page:The Kinematics of Machinery.djvu/540

 518 KINEMATICS OF MACHINERY.

then form that subdivision of the directiug-gear which we have called the supply. The feeding apparatus is here a direct and single-acting ratchet train, consisting of the chain (C'^P^)i with suction and delivery-valves as ratchet- and click-pawls. These are raised and dropped at the right instants by the fluid ratchet, the water. The whole arrangement forms therefore a second ratchet train which differs from the leading train in being single-acting and in requiring no director. The latter peculiarity, however, we may neglect ; for we could if we chose use a slide-valve worked by an eccentric instead of the automatic valves. In this case we should have in our machine two ratchet-trains, a reversed ratchet-train for the leading train, a direct one for the supply, both fitted with suitable directing-gear. In the case actually before us there are the additional differences that the one is single- and the other double- acting, and also that the pressure- organ is gaseous in the one and liquid in the other. These differences, however, we may suppose also to be removed, and the question then presents itself, why is the one ratchet-train reversed and the other direct, although both work with the same pressure-organ, the water passing through the boiler ? This resolves itself into the general question of the con- ditions which determine whether a ratchet-train with directing gear be direct or reversed. The answer is, that it is direct if the effort in this direction exceed the resistance reversed in the oppo- site case. The main train (the piston and its connections) receives from the column of steam a reversed motion (considered as a ratchet-train), because the effort of the steam exceeds the resistance at the crank ; while the train constituting the pump is a direct one, because here the driving effort at the crank (the eccentric a") is greater than the resistance at the plunger.* If at any time the mean resistance at the crank-pin becomes greater than the mean effort on the piston, the machine runs backwards, and the ratchet-train as such becomes a direct one, forcing first stearn and then air drawn through the exhaust pipe through what had been the supply pipe. We have illustrations of this every day in the working of the locomotive.

The fact that the directing gear of the ratchet-train, as we have seen, possesses the property of acting either forwards or backwards

nected with it, elsewhere more fully. R.
 * I hope to be able to treat this interesting question, and others directly con-