48 KANT'S UNIVERSAL NATURAL HISTORY 



the sun as the centre ; and, along with the comets, which 

 likewise revolve around the sun, coming to it from 

 all sides and in very wide orbits, they constitute a 

 system which is called the Solar System, or the Planetary 

 World. The movement of all these bodies, since it is 

 circular and returns into itself, presupposes two forces 

 which are equally necessary in every theory of the 

 system. The one is a Propulsive Force, whereby they 

 would continue to move on in a straight direction at 

 every point of their curved course, and would go off into 

 the infinite did not another force constantly compel 

 them to leave that direction and to move in a curved 

 line, which has the sun as its centre. This second force, 

 as is undoubtedly established by geometry, is always 

 directed to the sun, and is, therefore, called the Falling 

 Force, the Centripetal Force, or Gravitation. 



If the orbits of the planets were exact circles, the very 

 simplest analysis of the composition of curvilinear move- 

 ments would show that a constant tendency to the centre 

 would be required to produce them. But although the 

 orbits of all the planets, as well as of the comets, are 

 ellipses in whose common focus the sun is situated, yet 

 the higher geometry, by the aid of Kepler's Analogy 

 (according to which the radius vector, or the line drawn 

 from the planet to the sun, always describes areas of the 

 elliptical path, which are proportional to the time of 

 movement), shows with infallible certainty that a force 

 must incessantly impel the planet towards the centre of 

 the sun during the whole of its revolution. This falling 

 force, which rules through the whole extension of the 

 planetary system and is directed to the sun, is thus an 

 established phenomenon of nature. And just as certainly 



