Page:The Kinematics of Machinery.djvu/521

 FORM- AND PLAGE-CHANGING MACHINES. 499

is suspended is here the receptor, for it is the part of the machine directly connected with the driving weight. Looking at the question somewhat more closely, however, we see that the case is similar to that of the loaded crane ( 130). We may suppose, that is, the weight to be removed, but the chain or cord to be lengthened sufficiently to make up the load to its former amount ; it alone would then be sufficient to work the clock. The weight, therefore, cannot be the driver, for it no longer exists, while the cord has not altered its nature, and cannot therefore have been the receptor in the -former case. It is evident, however, that the cord is a link of the kinematic chain, and is paired with the barrel, on which it is coiled when the clock is wound up. Machines driven by weights have then this in common with the prime-movers just described, that the body transmitting the driving effort to them forms itself a part, a link or an element, of the kinematic chain.

The springs used for driving watches and other small machines are, as we have already seen in 44, kinematic elements or links. Here again it is very difficult, if not impossible, to say which piece of the machine answers to the common definition of the receptor, but we always see distinctly that the piece through which the driving effort is introduced forms a part of the kinematic chainage of the machine.

While, however, all prime-movers possess this common feature, our investigations show us that from another point of view they divide themselves into two general classes. In all those machines which are driven by pressure-organs a change of form takes place in the latter as it traverses the pipes, ports, valves, buckets, etc. ; this may be carried to a very great extent, as in the steam-engine, and may be combined with more or less change of place also. With springs the change of form alone exists. With driving weights, on the other hand, the change of place only remains, the change of form has completely disappeared. We have here exactly the difference which we found before to exist in regard to the object of the machine, or as we may now say, in regard to the treatment of the work-piece. We may there- fore again distinguish form-changing and place-changing machines according to the changes undergone by the driving organ in doing its work. This conclusion is not

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