26 



THE ANIMAL AS A MACHINE. 



of work " is derived by induction from experiment 

 and observation," and confirmed by all experience. 



TJie Sources of Energy are : (i) Potential : (a) fuel ; {h) 

 food ; {c) head of water ; id) chemical forces. (2) 

 Actual : {a) air in motion ; {U) gravity in waterfalls ; {c) 

 tides. 



8. "Newton's Laws " follow directly from the gen- 

 eral law of persistence of energy, a corollary to which 

 may be stated thus : Change of energy can only be pro- 

 duced by the action of force, and by doing work. 

 Newton's Laws are : 



(1) A free body tends to continue in the state in 

 which it, at any instant, exists, either of rest or of uni- 

 form rectihnear motion. 



(2) All change of motion in a body free to move is 

 proportional to the force impressed, and is in the direc- 

 tion of that force. 



(3) The reaction of the body acted upon by the im- 

 pressed force is equal, and directly opposed, to that 

 force. 



Inertia is that property, observed in all bodies, in 

 consequence of the existence of which they are capable 

 of exhibiting the action of these law^s. The laws of 

 Newton themselves are all easily verified by experiment. 

 The Atwood Machine," illustrated in nearly all works 

 on physics, is constructed for this special purpose. 



While Newton's laws are readily verified by experi- 

 ment, the more general laws of energetics are accepted 

 simply as being in accordance with universal experience. 

 The generally accepted theory of the constitution of 

 matter being assumed as a premise, however, the 

 general laws of energy are all easily deducible from 

 Newton's laws. Thus, the first law is but a differ- 



