AND THEORY OF THE HEAVENS. 51 



which two forces, the more they approach equality, make 

 the orbit more like the circular figure; and the more 

 unequal they are, the weaker the propulsive force is in 

 relation to the central force, so much the more elongated 

 will be the orbit. Or, as it is called, it is so much the 

 more eccentric^ because the heavenly body approaches the 

 sun much nearer at one part of its path than at another. 

 As there is nothing in the whole of nature that is 

 balanced with the utmost exactness, no planet has a wholly 

 circular movement. But the comets diverge most there- 

 from; because the impulsion which has been impressed 

 upon them laterally, has been least proportionated to the 

 central force of their initial distance. 



In this treatise I shall very frequently use the ex- 

 pression : A Systematic Constitution of the Universe. In 

 order that no difficulty may be found in clearly under- 

 standing what is meant to be indicated by the phrase, 

 I will shortly explain it. All the planets and comets 

 which belong to our world, properly constitute a system 

 by the fact that they revolve around a common central 

 body. But I use this designation in a stricter sense, 

 looking at the more exact relations which their combina- 

 tion with each other has made so regular and uniform, 

 The orbits of the planets are related as nearly as 

 possible to a common plane, which is that of the equator 

 of the sun prolonged; and deviation from this rule is 

 found only at the utmost limit of the system, where all 

 movements gradually cease. When, therefore, a number 

 of heavenly bodies which are arranged round a common 

 centre, and move around it, have at the same time been 

 so limited to a certain plane that they have freedom to 



