[13] Phil. Trans. 1821.
[14] Mr. Gough in Manch. Mem. vol. v.
[15] The reader who is acquainted with the two theories of light, will perceive that though we have adopted the doctrine of the ether, the greater part of the arguments adduced would be equally forcible, if expressed in the language of the theory of emission.
[16] Or rather through the focal centre of the eye, which is always near the centre of the pupil.
[17] Laplace, Expos. du Syst. du Monde, p. 441.
[18] In this statement of Laplace, however, one remarkable provision for the stability of the system is not noticed. The planets Mercury and Mars, which have much the largest eccentricities among the old planets, are those of which the masses are much the smallest. The mass of Jupiter is more than two thousand times that of either of these planets. If the orbit of Jupiter were as eccentric as that of Mercury is, all the security for the stability of the system, which analysis has yet pointed out, would disappear. The earth and the smaller planets might in that case change their approximately circular orbits into very long ellipses, and thus might fall into the sun, and fly off into remote space.
It is further remarkable that in the newly discovered planets, of which the orbits are still more eccentric than that of Mercury, the masses are still smaller, so that the same provision is established in this case also. It does not appear that any mathematician has even attempted to point out a necessary connexion between the mass of a planet and the eccentricity of its orbit on any hypothesis. May we not then consider this combination of small masses with large eccentricities, so important to the purposes of the world, as a mark of provident care in the Creator?
[19] The eccentricity of a planet’s orbit is measured by taking the proportion of the difference of the greatest and least distances from the sun, to the sum of the same distances. Mercury’s greatest and least distances are as two and three; his eccentricity, therefore, is one-fifth.
[20] The stability of the axis of rotation about which the earth revolves, has sometimes been adduced as an instance of preservative care. The stability, however, would follow necessarily, if the earth, or its superficial parts, were originally fluid; and that they were so is an opinion widely received, both among astronomers and geologists. The original fluidity of the earth is probably a circumstance depending upon the general scheme of creation; and cannot with propriety be considered with reference to one particular result. We shall therefore omit any further consideration of this argument.
[21] Airy on Encke’s Comet, p. 1, note.