252 KINEMATICS OF MACHLNERY.
required, its object being to show the mutual relation between two or more elements of "a mechanism ; whether two neighbouring elements be paired or linked together: if the latter, what their relative geometric position is ; whether a link be fixed or movable its relation, that is, to surrounding space and so on. Signs for these purposes we shall call symbols of relation.
The more exactly and explicitly the signs explain the kinematic elements and their chaining the better will they serve the purpose for which they are intended. We shall, however, be content with a certain degree of completeness in order to avoid diffuseness ; in all cases, however, the signs will express the real and general nature of the thing symbolized.
55. Class or Name-Symbols.
In choosing symbols to indicate the class to which, considered kinematically, a particular body belongs, we shall follow the example set us in chemical notation, and use Eoman capital letters, making these where possible the initial letter of the name of the class. The following twelve signs will be used to stand for the bodies whose names are placed after them :
Screw, G Sphere (Globe),
R Solid of Kevolution (Eevolute), A Sector or sweep (Arc),
P Prism, Z Tooth or projection,
C Cylinder, V Vessel or chamber,
K Cone (Konus), T Tension-organ,
H Hyperboloid, Q Pressure-organ.
It may appear remarkable that the number of classes of elements is so small. In fact, however, forms which can be called kinematic all lie within such a limited circle that a greater number of signs is not required, and it is advisable in all cases to be content with as few signs as possible. The letters have been chosen with care so as so far as possible to suggest the form for which they stand, and also to be available in other European languages than our own. I can also say from experi- ence that the recollection of the signs is no great tax upon the memory.
