256 INDUCTION OF 



The motions of the heavenly bodies having been 

 treated as if occurring in a vacuum, it has become 

 necessary to assume a special impulse at right angles 

 to the gravitating motion of the planets. But if the 

 planets be conceived as gravitating from any requi- 

 site distance through a plenum or resisting medium, 

 their motion towards the sun, being turned at right 

 angles, the centrifugal force is no longer a hypo- 

 thesis, but a demonstrable result. 



Or the facts may be regarded from another point 

 of view. Conceive a planet, at the aphelion, gravi- 

 tating towards the sun, but under such a resistance 

 that its motion assumes the actual curve of its orbit. 

 Under these conditions it falls by virtue of its gra- 

 vitation not to the sun, but beyond it. When it 

 reaches its perihelion, it will have attained a mo- 

 mentum of motion which would carry it through a 

 precisely corresponding curve, in opposition now to 

 gravity, to the aphelion again the point from which 

 it started. This is the motion of a pendulum start- 

 ing from the zenith and returning to the zenith, but 

 with its point of gravitation within, instead of ex- 

 ternal to, its circuit. But a planet thus oscillating 



