OF THE LAWS OF MOTION. 155 



if bodies had a natural tendency to move slower and slower, according 

 to a rate depending on the time elapsed. 



But if such cause existed, its effects ought to be considered sepa- 

 rately ; and it would still be requisite to assume the permanence of 

 the same velocity, as the first law of motion ; and to obtain, in addi- 

 tion to this, the laws of the retardation depending on the time. 



Whether there is any. such cause of retardation in the actual 

 motions of bodies, can be known only by a reference to experience; 

 and by such reference it appears that there is no such cause of the 

 diminution of velocity depending on time alone; and therefore that 

 the first law of motion may, in all cases in which bodies are exempt 

 from the action of external forces, be applied without any addition or 

 correction depending upon the time elapsed. 



It is not here necessary to explain at any length in what manner 

 we obtain from experience the knowledge of the truth just stated, that 

 there is not in the mere lapse of time any cause of the retardation of 

 moving bodies. The proposition is established by shewing that in all 

 the cases in which such a cause appears to exist, the cause of retar- 

 dation resides in surrounding bodies and not in time alone, and is 

 therefore an external force. And as this can be shewn in every in- 

 stance, there remains only the negation of all grovind for the assump- 

 tion of such a cause of retardation. We therefore reject it altogether. 



Thus it appears that in proving the first law of motion, we obtain 

 from our conception of cause the conviction that velocity will be 

 uniform except some cause produce a change in it ; but that we are 

 compelled to have recourse to experience in order to learn that time 

 alone is not a cause of change of velocity. 



8. I now proceed to the second Law : — that when a force acts 

 upon a body in motion, the effect is the same as that which the same 

 force produces upon a body at rest. 



This law requires some explanation. How is the effect produced 

 upon a moving body to be measured, so that we may compare it with 



