On the subject of the Solar System and Astronomy I will dwell for a few moments, as here--in this great example of the perfection of Modern Science--we have again a case apparently most adverse to my contention. The generalisations by which Newton established the nature of the planetary orbits has been a wonder to succeeding generations; the positions of the planets can be foretold, eclipses can be calculated with amazing accuracy. Yet every tyro in Mathematics knows that the equations which give these results can only be solved by what is called "neglecting small quantities"--that is, the problems cannot be solved in their entirety, but by leaving out certain terms and elements, which do not appear important, a solution can be approached. And naturally it has been an important point to show that these small quantities _may_ be safely neglected. In the case, for instance, of the orbits of the planets round the sun, and of the moon round the earth, it was for a long time taken as proved that the small variations in the shape and position of each elliptic orbit would never be accompanied by any permanent increase or diminution in its _size_--that is, that the _mean distances_ of the planets from the sun, and of the moon from the earth, would always remain within certain limits. Of late years however Professor George Darwin, taking up one of these poor little neglected quantities in the theory of the moon, found that it indicated after all very vast and very permanent, though of course very slow, changes in her mean distance from the earth; so that now it appears probable that the Moon's true orbit, instead of being a limited ellipse, is a continually though gradually enlarging Spiral, which may some day carry the Moon to a great distance from the earth. If an eclipse were calculated for twenty years in advance on the Elliptic theory or the Spiral theory, it would probably--so slow would be the divergence--make no perceptible difference; but in a hundred centuries the two theories would lead to results utterly different. Thus the certitude of Astronomy as a Science arises largely from the fact that our _times_ are so brief compared with Celestial periods. The proper periods of Celestial changes are to be reckoned by thousands, perhaps millions, of years; but we, ignoring _that_ aspect of the problem, fix our observations on one little point of time, and are quite satisfied with the result!