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in returning to the inside, and assuming in its progress every 

 various degree of distance from the centre of its orbit, in pro- 

 portion to the amount of progression. This ever varying change 

 of its relation to the orbit's centre is the result of a forward 

 movement on the part of the revolving body ; but neither does 

 the axis of the revolving body ever change its distance from the 

 orbit's centre, nor does the centre itself of the orbit ever change 

 its place at all ; consequently, it must be the point of the re- 

 volving body's circumference that changes its place, seeing that 

 its relation to the two invariable centres is constantly changing. 

 The axis does not revolve about it, so as to get between it and 

 the centre of the orbit ; the centre of the orbit does not revolve 

 about the axis of the planet, so as to fall behind the changing 

 point of circumference ; what remains therefore but to say, that 

 it is the changing point itself that revolves about the axis of its 

 own body, while that axis revolves about the immovable centre 

 of its orbit? When, at the second quarter of an orbital revolu- 

 tion, the seemingly invariable point of circumference is outside 

 of the orbit, farther distant from the orbit's centre, by a whole 

 diameter of its own body than it was at starting, how could it 

 possibly have got into that position, unless it had performed half 

 an orbital revolution, or half a rotation, round the moving axis? 

 It is no doubt true that it has not rotated under the impulse of 

 an intrinsic rotatory power acting upon it in a circular direction. 

 It {i. e. the point of circumference in question,) in common with 

 every other point of the circumference, and generally of the 

 planet's mass, is carried into the respective and successive posi- 

 tions which it and they occupy by virtue of the centrifugal force 

 that carries the body through the circumference of its orbit ; 

 and it is the extrinsic centripetal force, acting from the centre 

 of the orbit, that pulls continually the revolving body's axis, and 

 makes it deflect from the straight line it would follow, and, by 

 so pulling it, causes it continually to leasee behind the most ad- 

 vanced points of its circumference, whatever these may be for 

 the time ; and the ultimate result is, that every point of circum- 

 ference in succession is left behind in its turn by the axis^ till the 

 point which started in one with the radius* vector is again in one 

 with it, after having performed a complete circuit round the 

 axis, just as if, all the time, its motion forward had been retard- 

 ed by friction or other obstacle acting upon it from the outside 

 of the orbit, and pulling or keeping the circumference back, 

 while the axis moved forward ; and this circular movement of 

 the circumference is most unquestionably rotation^ or turning 

 round about the axis, though it have been caused by the move- 

 ment of the axis itself, that movement of the axis being in a di- 



