THE NEWTONIAN PHILOSOPHY, 521 



centrifugal force ; but it muft be proved ; and the ftrider proof will 

 be neccflary, that it appears to be contrary to common fenfe and ob- 

 fervation. 



2.do, Even where a motion is produced by powers operating in dif- 

 ferent directions, it is perfedly fimple ; the motion I mean is that in 

 the diagonal of the parallellogram, of which the two fides are the lines 

 of the diredion of the two powers adding upon the body. But, in this 

 cafe, where there is but one power that produces the motion, and that 

 ading internally, and upon every particle of the body, not upon the 

 turface only, as Dr Clarke has well obferved, there is ftill lefs reafon 

 for fuppofing that the motion fhould not only be combined, but fo in- 

 tricately combined, as to confift of three feveral motions ; one, by 

 which the body moves forward in the circle ; another, by which it in- 

 clines to the center ; and a third, by which it tends to fly off in a tan- 

 gent ; and all thefe feveral tendencies ading upon the body at the fame 

 time, and in every inftant of its movement. Is it not much more na- 

 tural to fuppofe, that the motion is as fimple as the power that pro- 

 duces it, or as the line which it defcribes, and which, by its nature, 

 is as fimple, and as little compounded, as a flraight line ? 



But, pioy To come clofer to the point, it is a principle of mctaphy- 

 fics, which cannot be difputed, that nothing can exifl without a caufe. 

 Now, where there is neither projedion nor gravitation, what is there 

 that can produce this double tendency in the body to and from the 

 center ? In the motion of projediles on earth, where all the three 

 tendencies above mentioned are undoubtedly combined, it is eafy to 

 account for each of them. It is the force imprefled by the impulfe of 

 body, which makes the body go forward in its courfe ; and it would 

 go forward in a flraight line, but for the power of gravitation, which 

 bends it from the flraight line towards the earth. And hence come 

 the two -other tendencies, the one centripetal, by which it iaclines 



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