16 THE HUMAN MOTOR 



rotation ; also, if CD transmits the movement to OM, the point 

 M at P or P' will be in the same straight line with CD and 

 could not pass those two positions, called dead points, except by 

 virtue of an acquired speed, because at these points, the speed 

 would otherwise be non-existant. 



10. Time, We have said that kinetics is the study of move- 

 ment, that is to say, of space and time, from which results the 

 idea of speed. The unit of tune, the second, is too long a period 

 for the analytic study of movement. Therefore both cinemato- 

 graphy and physiology have to employ means to register intervals 

 of time of T&TB of a second and sometimes but rarely- of T UTJU 

 of a second. These will be described later on (Methods, Book v., 

 198). 



11. The Study of Forces. Movement is the only phenomenon 

 which can be understood by observation ; it can be studied, and 

 its laws determined without having recourse to any other hypo- 

 thesis. As for the fundamental cause of movement, it appears 

 to be a question of metaphysics, rather than of science. When, 

 however, we are dealing with a point or a material system, the 

 immediate cause which produces the movement is called force ; 

 and when the movement is uniform it is again force which can 

 modify the motion. In fact, the given body would remain 

 stationary, or in its uniform movement, if it were not subjected 

 to some other exterior action or force. It would persist in in- 

 action through what is called inertia. 



Therefore the principle of inertia introduces but not neces- 

 sarily the idea of force. 



We say : not necessarily, because eminent mathematicians 

 have been able to dispense with it in all the developments of 

 mechanical laws. For instance, Kirchhoff in 1877, and Hertz, 

 in 1894. Carnot wrote even in 1803, " What can be understood, 

 in the precise language of mathematics by a force, that is to say, 

 by a cause double or triple of another ? . . . . Are the causes, 

 the will or the physical constitution of a man or an animal, 

 which by its action, gives birth to movement ? But what is a 

 will double or triple of another will ? " ( x ) 



An ever-present example of force is given by the action of the 

 earth on the bodies on its surface, the force of gravity ; bodies are 

 drawn by the earth and fall towards it vertically by a uniformly 

 accelerated movement, by virtue of a special force ; their acceler- 

 ation is constant in the same place ; it is represented by the 

 letter g, and it is about equivalent to 9 '81 m , that" is to say, the 

 acceleration of a falling body is 9'81 m at the end of a second. 



( l ) Lazare Carnot, Principes Fondamentaux de I'Equilibre ft du Mouve- 

 ment, 1803; Preface, p. *. 



