118 



would ever reach the centre, to which its motion, in no part of 

 its visible course, was ever directed ? " 



If it is true, that a body thrown up at a certain angle, say 45°, 

 will descend again at an angle of 45°, it follows that if thrown 

 sufficiently high it would escape the earth altogether, and pos- 

 sibly revolve round it : but what is the fact ? It is, that bodies 

 do not return at the same angle. Suppose a stone was thrown up 

 at an angle of 45° with a force which would carry it to an ele- 

 vation of 500 yards, it would on its near approach to the earth 

 descend almost perpendicular. It commences with its maximum 

 angular force, which exceeds that of the radial attraction of the 

 earth proportion ably to the sides of the parallelogram of which 

 its path is a compound ; at its greatest elevation the radial at- 

 traction and the impulsive force are in a state of equilibrium ; 

 as the latter rapidly decays, the former being constant and ac- 

 cumulating, the stone returns, and the angular or tangential 

 force becomes evanescent, until at length the stone is left to the 

 sole action of the radial attraction of the earth, and therefore 

 must proceed towards the centre. Those who may not feel dis- 

 posed to prove the above mathematically, or w T ho may prefer 

 having an ocular demonstration of the fact, may have an illus- 

 tration in the path of a stream of water forced by a strong pump 

 at a given angle, when it will be observed that the curve of the 

 stream of water, instead of forming the same angle at each ex- 

 tremity, will be very different. 



We shall now proceed with the extract, to show the conse- 

 quence which follows from the assumed proposition. 



" What reason/' it is said, " have we to believe that it might 

 not rather circulate round it, as the moon does round the earth, 

 returning again to the point it set out from, after completing an 

 elliptic orbit, of Avhich the centre occupies the lower focus ? And 

 if so, is it not reasonable to imagine that the same force deflects 

 the moon at every instant from the tangent of her orbit, and 

 keeps her in the elliptic path, &c? .... It is on such an argument 

 that Newton is understood to have rested his law of gravita- 

 tion/' 



That the moon is retained in a circular path round the earth 

 by the earth's radial attraction, in the same manner as a balloon 

 is retained at a certain elevation by means of the intermediate 



