oil UOTlOVt. ^3 



motions of those bodies may be, when compared with the surrounding objects; 

 and these relations can only be preserved by its continuance in uniform recti- 

 linear motion. Tills law is also confirmed by its perfect agreement with all 

 experimental observations, although it is too simple to admit of an immediate 

 proof. For we can never place any body in such circumstances as to be to- 

 tally exempt from the operation of all accelerating or retarding causes; and 

 the deductions from such experiments as we can make, would require in ge- 

 neral, for the accurate determination of the necessary corrections, a previous 

 knowledge of the law which we wish to demonstrate. 



When, indeed, we consider the motion of a projectile, we have only to allow 

 forthedisturbingforceof gravitation, which so modifies the effect, tliat the bo- 

 dy 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 these planes would be indifferent 

 to it; it must therefore remain in their common intersection, which could only 

 be a straight line: so that by thus combining arguments Avith observation, we 

 may obtain a confirmation of the law of the rectilinear direction of undisturb- 

 ed motion, partly founded on direct experiments 'Its uniformity is however 

 still less subjected to immediate examination; ytf, from a consideration of the 

 natiue of friction and resistance, combined with the laws of gravitation, we 

 thay ultimately show the perfect Coincidence of the flieory with experiment. 

 The tendency of matter to persevere in this manner' in the state of rest or 

 of uniform rej^tilinear motion, is called its inertia. 



In all thicse cases it is of importahcfe to attend to the composition of motion, 

 or the joint effieCt of more than one motion existing at the same time. The 

 existence of two or more motions, at the sAme time, in the same body, is not 

 at first comprehended without some difficulty. It is in fact only a combina- 

 tion or separation of relations that is considered: in the same manner as by 

 Combining the relation of son to father, and brother to brother, we obtain, 

 the relation of 'nephew to uncle, so by combining the motion of a man walk- 

 ing in a ship, with the motion of the ship, we determine the relative velocity 

 of the man with respect to the earth's surface. It is, however, necessary, for 

 ascertaining these relations, to consider the affections of a space or surface in 



