Page:The Kinematics of Machinery.djvu/273

 KINEMATIC SYMBOLS. 251

For our purpose the representation of kinematic chains by symbols, Babbage's method is of no service ; I therefore pass over Willis' attempts to make it more useful by certain alterations.*

54 Nature of the Symbols required.

The object of the kinematic notation which we wish to form is, like that of mathematical symbols, to express certain operations performed with, or supposed to be performed with, the bodies indi- cated by signs or otherwise ; partly also its province is similar to that of chemical notation, for it must afford information, and indeed somewhat full information, as to the quality of the thing named. The symbols for kinematic bodies must not, therefore, be in them- selves meaningless like those of Mathematics, where different letters indicate only the variations in magnitude of known, and so far as their measurability is concerned similar, things ; but each letter must stand, as in Chemistry, for a particular class of bodies, the differences between the classes being here their geometrical pro- perties. The letter must therefore stand for the name of the body that is, of the kinematic element, the definite characteristics of which are sufficiently indicated by that name. The letter used in this way we shall call the class- or name-symbol of the element.

The sign for the general name of a kinematic element, as e.g. the sign for " screw, " " revolute/'^ " prism," and so on, is seldom suf- ficient by itself. Most frequently some further indication is re- quired as to the form of the body, as for example whether the screw be external or internal, that is whether the screw-spindle or the nut be meant. The geometrical basis figure is the same in both cases, but there is a great difference between the forms in which it is used. Signs serving to indicate this more exactly, which will be used in connection with the name-symbols, we shall call form- symbols.

In addition to these two classes of symbols, a third kind is

intended that the letters and symbols should be put on the drawings themselves, as Prof. Reuleaux suggests.


 * Willis : Principles of Mechanism, p. 343. (2nd. Ed. 292.)

t See page 91.