24 INVARIABLE PLANE. SECT. III. 



given time, by the projection of its radius vector about the 

 common centre of gravity of the whole, the sum of all these pro- 

 ducts will be a maximum (N. 81). La Place found that the 

 plane in question is inclined to the ecliptic at an angle of nearly 

 1 34' 15", and that, in passing through the sun, and about 

 midway between the orbits of Jupiter and Saturn, it may be 

 regarded as the equator of the solar system, dividing it into two 

 parts, which balance one another in all their motions. This 

 plane of greatest inertia, by no means peculiar to the solar sys- 

 tem, but existing in every system of bodies submitted to their 

 mutual attractions only, always maintains a fixed position, 

 whence the oscillations of the system may be estimated through 

 unlimited time. Future astronomers will know, from its im- 

 mutability or variation, whether the sun and his attendants are 

 connected or not with the other systems of the universe. Should 

 there be no link between them, it may be inferred, from the 

 rotation of the sun, that the centre of gravity (N. 82) of the 

 system situate within his mass describes a straight line in this 

 invariable plane or great equator of the solar system, which, 

 unaffected by the changes of time, will maintain its stability 

 through endless ages. But, if the fixed stars, comets, or any 

 unknown and unseen bodies, affect our sun and planets, the 

 nodes of this plane will slowly recede on the plane of that im- 

 mense orbit which the sun may describe about some most distant 

 centre, in a period which it transcends the power of man to de- 

 termine. There is every reason to believe that this is the case ; 

 for it is more than probable that, remote as the fixed stars are, 

 they in some degree influence our system, and that even the 

 invariability of this plane is relative, only appearing fixed to 

 creatures incapable of estimating its minute and slow changes 

 during the small extent of time and space granted to the human 

 race. " The development of such changes," as M. Poinsot justly 

 observes, " is similar to an enormous curve, of which we see so 

 small an arc that we imagine it to be a straight line." If we 

 raise our views to the whole extent of the universe, and consider 

 the stars, together with the sun, to be wandering bodies, revolv- 

 ing about the common centre of creation, we may then recognise 

 in the equatorial plane passing through the centre of gravity of 

 the universe the only instance of absolute and eternal repose. 

 All the periodic and secular inequalities deduced from the law 



