ASTRONOMY. 227 



could not be obtained. Here then are clearly shown regu- 

 lation and design. A law both admissible and convenient 

 was to be obtained : the mode chosen for obtaining that law 

 was by making each particle of matter act. After this choice 

 was made, then farther attention was to be given to each 

 particle of matter, and one, and one only particular law of 

 action to be assigned to it. No other law would have an- 

 swered the purpose intended. 



(*) 2. All systems must be liable io perturbations. And 

 therefore to guard against these perturbations, or rather to 

 guard against their running to destructive lengths, is per- 

 haps the strongest evidence of care and foresight that can 

 be given. Now we are able to demonstrate of our law of 

 attraction, what can be demonstrated of no other, and what 

 qualiiies the dangers which arise from cross but unavoidable 

 influences, that the action of the parts of our system upon 

 one another will not cause })ermanently increasing irregular- 

 ities, 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 prop- 

 erties concur, viz. that the force shall be inversely as the 

 square of the distance ; the masses of the revolving bodies 

 small, compared with that of the body at the centre ; the or- 

 bits 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 performing. 



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

 planets, Jupiter and Saturn, had moved in lower spheres, 

 their influences would have had much more effect as to 

 disturbing 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 exact circles, in which case, no change of dis- 

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



