66 OF UNDISTURBED MOTION. 



premised, and principally from the consideration of the 

 relative nature of motion, and the total deficiency of any 

 criterion of absolute motion : it is also confirmed by its 

 perfect agreement with all experimental observations, al- 

 though it is too simple to admit of an immediate proof. 

 For we can never placie any body in such circumstances, as 

 to be totally exempt from the operation of all accelerating 

 or retarding causes ; and the deductions from such expe- 

 riments, as we can make, would require, in general, for 

 the accurate determination of the necessary corrections, a 

 previous assumption of the law which we wish to demon- 

 strate. 



Scholium 3. When, indeed, we consider the motion 

 of a projectile, we have only to allow for the disturbing 

 force of gravitation, which so modifies the effect, that the 

 body deviates from a right line, but remains in the same 

 vertical plane; whence we may infer, that, in the absence 

 of the force of gravitation, the body would continue to 

 move in every other plane in which its motion began, as 

 well as in the vertical plane, since in that case all planes 

 would be indifferent to it ; it would, therefore, necessarily 

 remain in their common intersection, which could only be a 

 straight line : so that, by thus combining argument with 

 observation, we may obtain a confirmation of the law of 

 the rectilinear direction of undisturbed motion, founded in 

 great measure on direct experiment. The uniformity of 

 undisturbed motion, is, however, still less subjected to 

 immediate examination ; yet, from a consideration of the 

 nature of friction and resistance, combined with the laws 

 of gravitation, we may ultimately show the perfect coinci- 

 dence of the theory with experiment. 



Scholium 4. The tendency of matter to persevere in 

 the state of rest, or of uniform rectilinear motion, is called 



