512 KINEMATICS OF MACHINERY.
from the shingling rolls, and so on. We shall call the mechanism which plays this striking part in every machine its main or leading train. The names we have just cited show that in practice special importance has already attached to the mental separation of this train from the whole machine; indeed the establishment of the foregoing generalisation leads directly to this special distinction,
It is in the design of the leading train of a machine that we meet with those requirements which the old theory has attempted to satisfy by receptor and tool. If either or both of these exist at all they will form part of the main train, and can be -treated by themselves if it be wished. I believe I may say that the practical mechanic has very seldom troubled himself about the exact determination of the receptor, while the whole construction of what we have called the main train flashes at once before him so soon as the name of the machine is pronounced. This makes it the more necessary that we should endeavour to ascertain theoretically what is included in this idea.
In our common direct-acting engine the leading- train is a ratchet- train, formed from piston and chamber with valves, and the slider- crank train (C"P- L )1-. In a wharf crane of the usual kind the main-train is running-gear (p. 498) formed of the chain, barrel and spur-gearing; in a heckling machine it may be a pair of heckling rollers with their driving mechanism ; in the spinning- machine it is the draw-frame and spindles and their driving mechanisms, and so on.
There are many machines in which we find, as in the one last mentioned, that the main-train consists of several parts, or that several main-trains are united, each acting at its own proper time. In some cases this action is periodic, and frequently also, where the leading train is single only, we find a periodic sequence of single changes of, motion occurring, governed by special me- chanisms. These mechanisms may be considered as forming a group by themselves. In many pressure-organ machines they are repre- sented by the valve-gear, but as they occur in many other cases where there are no valves, we shall include them generally under the name of the director or directing-gear of the machine. The director is therefore the apparatus by which the motions of the machine are caused to succeed each other in their required order.
