110 DISSERTATION SECOND. [parti. 



a certain velocity, as to give to double the weight the 

 half of that velocity, and so on in proportion, the effect 

 being always measured by the weight multiplied into the 

 velocity which it receives. He could hardly be ignorant 

 that this proposition had been already slated by Galileo, 

 but he has made no mention of it. He, indeed, always 

 affected a disrespect for the reasonings and opinions of the 

 Italian philosopher, which has done him no credit in the 

 eyes of posterity. 



The theory of motion, however, has in some points been 

 considerably indebted to Descartes. Though the reason- 

 ings of Galileo certainly involve the knowledge of the dis- 

 position which matter has to preserve its condition either of 

 rest or of rectilineal and uniform motion, the first distinct 

 enunciation of this law is found in the writings of the 

 French philosopher. It is, however, there represented, 

 not as mere inactivity, or indifference, but as a real force, 

 which bodies exert in order to preserve their state of rest 

 or of motion, and this inaccuracy affects some of the rea- 

 sonings concerning their action on one another. 



Descartes, however, argued very justly, that all motion 

 being naturally rectilineal, when a body moves in a curve, 

 this must arise from some constraint, or some force urging 

 it in a direction different from that of the first impulse, and 

 that if this cause were removed at any time, the motion 

 would become rectilineal, and would be in the direction of 

 a tangent to the curve at the point where the deflecting 

 force ceased to act. 



Lastly, He taught that the quantity of motion in the uni- 

 verse remains always the same. 



The reasoning by which he supported the first and se- 

 cond of these propositions is not very convincing, and 

 though he might have appealed to experience for the truth 

 of both, it was not in the spirit of his philosophy to take 



