Page:The Kinematics of Machinery.djvu/261

 has now just made its first entry. Here also, as everywhere else, force-closure is given up to make way for the more complex but kinematically more complete closure by pairs and chains.

In addition to this internal difference between the modern and the ancient machine, there is another very important but external difference. This lies in the improved construction of the single parts of the machine,—the links, that is, of the kinematic chains of which it consists. The introduction in the last century of cast iron in place of wood led gradually to the making of beams, wheels, levers, frames, etc., each in as few pieces as possible, when practicable at last each out of a single piece. In our own time cast steel has commenced to act in the same way upon construction in wrought iron. In designing machinery, increasing care is taken that the intensities of the latent and the sensible forces correspond, and the dimensions given to each part are calculated with precision. These dimensions are, in consequence, much less than they once were; and this has caused what is essentially a decrease in the number of parts, or more correctly of, which has extraordinarily simplified the external appearance of the machine. For this reason the modern machine often appears more simple than the ancient one, although in reality it is generally far more complicated. The old wharf cranes, for example, by which but very moderate loads could be lifted, were much more striking in appearance than our slighter-looking machines, which are really so much stronger, as well as so much more complex in their construction. The same thing would be noticed in comparing the old and the new pumping-machinery, mill-work, steam-engines, etc. This important external simplification of the machine—a process which is still continually going on—has been the means of making its actual construction more and more easy. It must not be forgotten that this increased facility of execution depends upon the enormous capabilities for work which we have stored up in the resources of our workshops. It is the interest of this capital that we find in the external simplification of the machine. It must not be allowed to conceal from us the simultaneous increase of internal complexity. That there are limits to this increase, pointed out by kinematic conditions and already in some instances nearly approached, we shall find as we proceed. At present I cannot enter into this very important question.