AbTRONOMY. 259 



voidable iiifl'iences — that the action of the parts of our 

 system upon one another will not cause permanently in- 

 creasing irregularities, but merely periodical or vibratory 

 ones ; that is, they will come to a limit and then go back 

 again. This we can demonstrate only of a system in which 

 the following properties concur, namely, that the force shal] 

 be inversely as the square of the distance ; the masses ol 

 the revolving bodies small, compared with that of the body 

 at the centre ; the orbits not much inclined to one another ; 

 and their eccentricity little. In such a system the grand 

 points are secure. The mean distances and periodic times, 

 upon which depend our temperature and the regularity of 

 our year, are constant. The eccentricities, it is true, will 

 still vary ; but so slowly, and to so small an extent, as to 

 produce no inconveniency from fluctuation of temperature 

 and season. The same as to the obliquity of the planes of 

 the orbits. For instance, the inclination of the ecliptic to 

 the equator will never change above two degrees, out of 

 ninety, and that will require many thousand years in per- 

 forming. 



It has been rightly also remarked, that if the great plan- 

 ets Jupiter and Saturn had moved in lower spheres, their 

 influences would have had much more effect as to disturb- 

 ing the planetary motions than they now have. While they 

 revolve at so great distances from the rest, they act almost 

 equally on the sun and on the inferior planets ; which has 

 nearly the same consequence as not acting at all upon 

 either. 



If it be said, that the planets might have been sent round 

 the sun in exLct circles, in which case, no change of dis- 

 tance from the centre taking place, the law of variation ol 

 the attracting power would have never come m question, 

 one law would have served as well as another ; an answer 

 to the scheme may be drawn from the consideration of these 

 same perturbing forces. The system retaining in other re- 

 spects its present constitution, though the planets had been 



