881 



projection, which is continually diminished thereby. 

 And as it has no tendency to recover this, the smallest 

 continued resistance will at last quite exhaust its power, 

 though originally ever so great. Thus every projectile 

 on the earth, however great the projecting force may be 

 at setting out, is continually retarded till it rests in the 

 direction of a parabolic curve. 



Sir Isaac seems not to have reflected on this circum- 

 stance of gravitation and projection, that the one retains* 

 its whole tendency to motion, whether it be retarded or 

 stopped, while the other always loses as much power as 

 it meets with opposition. Neither in balancing these 

 powers, does he seem to have reflected on that obvious 

 truth : that every alteration in the direction of a moving 

 projectile, destroys so much of its motion, which cannot 

 be repaired, but by a continued action of the first 

 moving cause. 



Philosophers illustrate the joint effect of centripetal 

 and centrifugal force, in making bodies move in a circle, 

 by the experiment of casting round a weight, suspended 

 by a string in one's hand. But this illustration contains 

 a palpable deception. For the power of the string re- 

 straining the body from flying off in a strait line, bears 

 no analogy to a power actutually drawing a moving body 

 towards iis centre of motion. The string resists its 

 ftylug off, but has not the least tendency to draw it nearer. 

 And whatever is the cause of the revolution of the pla- 

 nets, it must be some cause which simply resists their 

 ying off into eccentric motions. It cannot in the nature 

 of things be one, which is uniformly drawing them into 

 their centre of motion. 



But suppose both gravitation and projection had the 

 same property of still retaining their original tendency to 

 their respective motions, however they were retarded : 

 still it is impossible that these two powers, acting by 

 immutable laws, can move an orb any otherwise than in 

 a circle : whereas all the planets are allowed by philoso- 

 phers themselves to move in ellipsis. 



These powers moving an orb hi the figure of an ellip- 

 sis, must H9 less than four times vary the proportion of 



