1. The science which investigates the action of Force is called, by the most logical writers, Dynamics. It is commonly, but erroneously, called Mechanics; a term employed by Newton in its true sense, the Science of Machines, and the art of making them.

2. Force is recognized as acting in two ways:

1° so as to compel rest or to prevent change of motion, and

2° so as to produce or to change motion.

Dynamics, therefore, is divided into two parts, which are conveniently called Statics and Kinetics.

3. In Statics the action of force in maintaining rest, or preventing change of motion, the ‘balancing of forces,’ or Equilibrium, is investigated; in Kinetics, the action of force in producing or in changing motion.

4. In Kinetics it is not mere motion which is investigated, but the relation of forces to motion. The circumstances of mere motion, considered without reference to the bodies moved, or to the forces producing the motion, or to the forces called into action by the motion, constitute the subject of a branch of Pure Mathematics, which is called Kinematics, or, in its more practical branches, Mechanism.

5. Observation and experiment have afforded us the means of translating, as it were, from Kinematics into Dynamics, and vice versâ. This is merely mentioned now in order to show the necessity for, and the value of, the preliminary matter we are about to introduce.