Page:The Kinematics of Machinery.djvu/63

 Rh Summarising this section, we see that "Practical Mechanics" has been subdivided into—


 * The study of Machinery in general.
 * The special or theoretical study of Machinery.
 * The study of Machine-design.
 * The study of pure Mechanism.

For the understanding of the nature of machines the last-named science is evidently as important as the three first; indeed in many respects it must stand first and prepare the way for them, and on this account single sections of Kinematics are often included in all three. The union of the three last sciences is necessary that the machine may be completely understood, the first having pointed out its existence and treated it teleologically. All four interact continually; only as a whole do they furnish the practical mechanist with complete solutions to all the problems of his work.

We must now proceed to establish the general principles of kinematic procedure, in order to gain a standpoint from which to survey generally the method of solving our problem. The ideas above developed concerning the essential nature of machine-systems will serve as an introduction to this. Those parts of a machine transmitting the forces by which the moving points are caused to limit their motions in the definite and required manner must be bodies of suitable, the moving points themselves must belong also to similar bodies. In the machine, consequently, the moving bodies are prevented, by bodies, from making any other than the required motions. This contact also, if the problem is to be entirely solved, must take place continually, which presupposes the possession of certain properties by the bodies in contact. In proceeding to examine these properties more closely, we shall