Illustrations of the Huttonian Theory of the Earth

by John Playfair

Note xx. § 118.

Inequalities in the Planetary Motions.

385. The assertion that, in the planetary motions, we discover no mark, either of the commencement or termination of the present order, refers to the late discoveries of La Grange and La Place, which have contributed so much to the perfection of physical astronomy. From the principle of universal gravitation, these mathematicians have demonstrated, that all the variations in our system are periodical; that they are confined within certain limits; and consist of alternate diminution and increase. The orbits of the planets change not only their position, but even their magnitude and their form: the longer axis of each has a slow angular motion; and, though its length remains fixed, the shorter axis increases and diminishes, so that the form of the orbit approaches to that of a circle, and recedes from it by turns. In the same manner, the obliquity of the ecliptic, and the inclination of the planetary orbits, are subject to change; but the changes are small, and, being first in one direction, and then in the opposite, they can never accumulate so as to produce a permanent or a progressive alteration. Thus, in the celestial motions, no room is left for the introduction of disorder; no irregularity or disturbance, arising from the mutual action of the planets, is permitted to increase beyond certain limits, but each of them, in time, affords a correction for itself. The general order is constant, in the midst of the variation of the parts; and, in the language of La Place, there is a certain mean condition, about which our system perpetually oscillates performing small vibrations on each side of it, and never receding from it far.[199] The system is thus endowed with a stability, which can refill the lapse of unlimited duration; it can only perish by an external cause, and by the introduction of laws, of which at present no vestige is to be traced.

[199] Exposition du Systéme du Monde, par La Place, Livre iv. chap. 6. p. 199. 2d edit.

386. The same calculus to which we are indebted for these sublime conclusions, informs us of two circumstances, which mark the law here treated of as an effect of wise design, to the entire exclusion both of necessity and chance. One of these circumstances consists in the planetary motions being all in the same direction, or all in consequentia as it is called by the astronomers. This is essential to the compensation and stability above mentioned:[200] had one planet circulated round the sun in a direction from east to west, and another in a direction from west to east, the disturbances they would have produced on one another's motion would not necessarily have been periodical; their irregularities might have continually increased, and they might have deviated in the course of ages from their original condition, beyond any limits that can be assigned.

[200] La Place, ibid.

The other circumstance, on which the stability of our system depends, is the small eccentricity of the planetary orbits, or their near approach to circles. Were their orbits very eccentric, an opening would be given to progressive change, that might so far increase, as to prove the destruction of the whole. But neither the movement of all the planets in the same direction, nor the small eccentricity of their orbits, can be ascribed to accident, since that either of these should happen by chance, in as many instances as there are planets, both primary and secondary, is almost infinitely improbable. Again, that any necessity in the nature of things should have either determined the direction of the planetary motions, or proportioned the quantity of them to the intensity of the central force, cannot be admitted, as these are things unavoidably conceived to be quite independent of one another. It remains, therefore, that we consider the laws, which make the disturbances in our system correct themselves, and by that means give firmness and permanence to it, as a proof of the consummate wisdom with which the whole is constructed.

387. The geological system of Dr Hutton, resembles, in many respects, that which appears to preside over the heavenly motions. In both, we perceive continual vicissitude and change, but confined within certain limits, and never from a certain, mean condition, which is such, that, in the lapse of time, the deviations from it on the one side, must become just equal to the deviations from it on the other. In both, a provision is made for duration of unlimited extent, and the lapse of time has no effect to wear out or destroy a machine, constructed with so much wisdom. Where the movements are all so perfect, their beginning and end must be alike invisible.