Page:The Kinematics of Machinery.djvu/262

 Putting in a few words the results of our examination in their relation to the fundamental idea laid down at the beginning of the chapter, we may say that the limitation of force-closure has essentially been the means by which machines have been made capable of better carrying out their own share of work. This limitation led gradually from the make-shift first attempts at machines to the accurately working pairs of elements and the simpler mechanisms. This at the same time creates the possibility, and becomes the cause, of further extension of the limits within which the machine acts,—of obtaining larger results by human intellect,—or as we expressed it before, of making the share of the machine a larger fraction of the whole problem.

The endeavours after this lead to the invention of new mechanisms, and in these again force-closure—which seems always to be nearest to our hands—is at first employed. This shows itself every day, especially in machines invented by workmen or others whose knowledge of their subject is merely empirical. Of such machines we have many; not unfrequently they have been pioneers to open up a new region. They contain such a combination of weights, springs, tappets, catches, stamps, fly-wheels and so on, clattering and jerking in their force-closed working, that they might be a little representation of all the steps in the development of the machine seen through a reversed telescope. The experienced and scientific designer sets them aside with a smile, and replaces them with accurately working elements. But in spite of his experience and knowledge, if the same man have to design am entirely new machine, he too will at first employ force-closure in many places where he might better have used pair-closure, and where in time he will use it. The Corliss valve-gear is a capital example of this; in its earliest form it was everywhere force-closed, and all the subsequent improvements have been unconsciously in the direction of the replacement of this by something better. In the  growth of the machine we thus see that the removal of force-closure is also continually going on, by restricting its employment within narrow limits, so distinctly that we cannot wish, nor indeed dare, to attempt to return again to its use.

We must not overlook the fact that to a certain extent the general development of the machine has hitherto gone on unconsciously, and that this unconsciousness which has characterised the