96 NOMOS. 



are called, and went on to explain by them the 

 whole scheme of celestial motion. The laws are 

 these. 



The first is, that a moving body will move with 

 uniform velocity in a straight line, and continue its 

 motion for ever, unless it is acted upon by some 

 external force. This is the necessary consequence 

 of the inertia of this body. Without inherent capacity 

 of action, indeed, this body remains for ever at rest 

 if at rest, and moves for ever in a straight line if in 

 motion and perfectly free to move. In itself the 

 body is purely passive. 



The second law is, that the change of motion which 

 is caused when a moving body is acted upon by any 

 force is the same as that which would have been 

 produced if the same force had acted upon the same 

 body during the state of rest. 



The third law is, that action and reaction are equal ; 

 or, in other words, that the mutual action of two 

 bodies upon each other is equal in degree and con- 

 trary in direction. 



In applying these laws to the explanation of the 

 movements of the heavenly bodies, it is assumed that 

 space is free from everything which can offer resist- 

 ance to the movements of these bodies, and that each 

 body was originally launched into its orbit with a 

 given velocity a velocity varying for each body 

 by a force which ceased to operate as force from that 

 moment. 



Moving in free space, and set in motion in this 

 manner, a planet, for example, tends to move for ever 

 in a straight line ; but instead of obeying this 



