Illustrations of the Huttonian Theory of the Earth

by John Playfair

383. It is true, that bodies which float in the water, when carried along on the tops of the waves towards a shelving beach, having acquired a certain velocity, are thrown farther in upon the land than the distance they would have floated to, if they had been simply sustained by the water. The depth of water, therefore, at the place where they take the ground, is not likely to be such as to float them again, and to carry them out towards the sea. They are, therefore, left behind; and this produces an appearance of a force impelling floating bodies towards the land, much greater and more general than really takes place.

These observations may serve to show, how unsound the principles are from which Mr Kirwan's conclusions are deduced: they are perhaps more than is necessary for that purpose: it might have been sufficient to observe, that the increase of land on the sea shore is limited, though the augmentation from the land is certainly indefinite, a proof that the diminution from the sea is constant and equal to the increase.

384. "Mariners," says Mr Kirwan, "were accustomed, for some centuries back, to discover their situation, by the kind of earth or sand brought up by their sounding plummets; a method which would prove fallacious, if the surface of the bottom did not continue invariably the same."[198]

[198] Geol. Essays, p. 440.

The fact here stated, that mariners, when navigation was more imperfect than it is now, had very frequent recourse to this method, and that they still use it occasionally, is very true. But from this, the only inference that can be fairly deduced is, that the changes at the bottom of the sea are very slow, and the variation but little; not merely from one year to another, but even from one century to another. The rules by which the mariner judged of his position from the quality of the earth which the lead brought up, and which were deduced no doubt from observations made at no very great distance of time, might be sufficient for his purpose, though a slow change had been all the while going forward. Such observations could at best have little accuracy, and could not be affected by small variations. It is the slowness of the change, that makes the experience of one age applicable, in this, as in innumerable other instances, to the observations of the next. If a long interval is taken, we will look in vain for the same uniformity of results. A pilot, who would at present judge of his position in the German Ocean, by comparing his soundings with those taken by Pytheas, (supposing them known) in his navigation of that sea, more than 2000 years ago, could hardly be expected to determine his latitude and longitude with great exactness; and I know not if the most zealous advocate for the immutability of the earth's surface, would be willing to trust his safety in a ship that was guided by such antiquated rules.

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.