THE HUMAN MOTOR. 



BOOK I. 

 GENERAL PRINCIPLES OF MECHANICS. 



CHAPTER I. 



1. General. A consideration of the general laws of mechanics 

 is an indispensable preliminary to the study of any machine 

 (animate or inanimate). It is only thus that, the problems in- 

 volved in equilibrium and movement can be properly appreciated. 



The subject of mechanics is generally divided under three 

 heads : 



1. Kinetics. (The study of movement in itself) . 



2. Statics. (The study of the equilibrium of forces). 



3. Dynamics. (The study of force in action). 



Such subdivision is both simple and logical. It does not ex- 

 clude consideration of all the physical properties of the bodies 

 under study. Mechanics in the practical department, to which 

 the term " Applied Mechanics " is given, specifically takes account 

 of all such properties, and especially of the deformation caused 

 by the application of forces. It further deals with the strength 

 of materials and the limiting stresses which solid bodies can 

 sustain. 



A clear understanding of the mechanism of human activity is 

 not possible without some general knowledge of the above laws. 

 The writer has therefore thought it necessary to devote the earlier 

 pages of this work to the subject of mechanics in general, both in 

 order to give completeness to the discussion and also to save the 

 reader the inconvenience of having frequently to refer to text- 

 books and other works of reference. 



2. (1). Kinetics. When we see a body change its place, we say 

 that it is moving and we imply that it has been stationary. In 

 reality, all movement is relative and is reckoned by comparison 

 of the body which moves, with another body, which serves as a 



