SKCT. HI. STABILITY OF THE SYSTEM. 21 



as well planets as satellites, in the same direction. 

 These secure the perpetual stability of the solar system 

 (N. 77). The equilibrium, however, would be de- 

 ranged, if the planets moved in a resisting medium 

 (N. 78) sufficiently dense to diminish their tangential 

 velocity, for then both the eccentricities and the major 

 axes of the orbits would vary with the time, so that the 

 stability of the system would be ultimately destroyed. 

 The existence of an ethereal fluid is now proved ; and 

 although it is so extremely rare that hitherto its effects 

 on the motions of the planets have been altogether in- 

 sensible, there can be no doubt that, in the immensity 

 of time, it will modify the forms of the planetary orbits, 

 and may at last even cause the destruction of our sys- 

 tem, which in itself contains no principle of decay, unless 

 a rotatory motion from west to east has been given to this 

 fluid by the bodies of the solar system, which have all 

 been revolving about the sun in that direction for un- 

 known ages. This rotation, which seems to be highly 

 probable, may even have been coeval with its creation. 

 Such a vortex would have no effect on bodies moving 

 with it, but it would influence the motions of those re- 

 volving in a contraiy direction. It is possible that the 

 disturbances experienced by comets which have already 

 revealed the existence of this fluid, may also, in time, 

 disclose its rotatory motion. 



The form and position of the planetary orbits, and the 

 motion of the bodies in the same direction, together with 

 the periodicity of the terms in which the inequalities 

 are expressed, assure us that the variations of the sys- 

 tem are confined within very narrow limits, and that, 

 although we do not know the extent of the limits, nor 

 the period of that grand cycle which probably embraces 

 millions of years, yet they never will exceed what is 

 requisite for the stability and harmony of the whole, for 

 the preservation of which every circumstance is so beau- 

 tifully and wonderfully adapted. 



The plane of the ecliptic itself, though assumed to be 

 fixed at a given epoch for the convenience of astronomi- 

 cal computation, is subject to a minute secular variation 

 of 45"-7, occasioned by the reciprocal action of the plan- 

 ets. But, as this is also periodical, and cannot exceed 



