of our system, which in itself contains no principle of decay.
Three circumstances have generally been supposed necessary to prove the stability of the system: the small eccentricities of the planetary orbits, their small inclinations, and the revolutions of all the bodies, as well planets as satellites, in the same direction. These, however, though sufficient, are not necessary conditions; the periodicity of the terms in which the inequalities are expressed is enough to assure us that, though 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 beautifully and wonderfully adapted.
The plane of the ecliptic itself, though assumed to be fixed at a given epoch for the convenience of astronomical computation, is subject to a minute secular variation of 45"·7, occasioned by the reciprocal action of the planets; but, as this is also periodical, and cannot exceed 3°, the terrestrial equator, which is inclined to it at an angle of about 23° 27' 34"·5, will never coincide with the plane of the ecliptic: so there never can be perpetual spring. The rotation of the earth is uniform;
