H2 DISSERTATION SECOND. [parti. 



have appeared the most decisive indications of errour. 

 How this escaped the penetration of a man well acquainted 

 with the harmony of geometrical truths, and the gradual 

 transitions by which they always pass into one another, is 

 not easily explained, and perhaps, of all his errours, is the 

 least consistent with the powerful and systematick genius 

 which he is so well known to have possessed. 



Thus, the obligation which the theory of motion has to 

 this philosopher, consists in his having pointed out the 

 nature of centrifugal force, and ascribed that force to the 

 true cause, the inertia of body, or its tendency to uniform 

 and rectilineal motion. 



The laws which actually regulate the collision of bodies 

 remained unknown till some years later, when they were 

 recommended by the Royal Society of London to the 

 particular attention of its members. Three papers soon 

 appeared, in which these laws were all correctly laid down, 

 though no one of the authors had any knowledge of the 

 conclusions obtained by the other two. The first of these 

 was read to the Society, in November, 1668, by Dr. Wal- 

 !is of Oxford ; the next by Sir Christopher Wren in the 

 month following, and the third by Huygens, in January, 

 1669. The equality of action and reaction, and the max- 

 im, that the same force communicates to different bodies 

 velocities which are inversely as their masses, are the 

 principles on which these investigations are founded. 



The ingenious and profound mathematician last men- 

 tioned, is also the first who explained the true relation 

 between the length of a pendulum, and the time of its least 

 vibrations, and gave a rule by which the time of the rec- 

 tilineal descent, through a line equal in length to the pen- 

 dulum, might from thence be deduced. He next applied 

 the pendulum to regulate the motion of a clock, and gave 

 an account of his construction, and the principles of it, in 



