six hours. Now, the wave, breaking upon the beach, represents periodic variations; and the tide represents secular variations. Both are strictly periodical, but the latter are called secular from the comparatively vast period necessary to effect a compensation. If we conceive the ends of the oval lake to be affected only by the small waves breaking on the beach, but the sides affected by tides, we shall have an exact representation of a planetary orbit. The small waves would not sensibly lengthen or shorten the lake, but the tides would, in a marked manner. Now, the length of a planet's orbit is subject only to these very small oscillations, and it may therefore be regarded as fixed or constant. The breadth of the orbit, however, is subject to secular variations, which require ages for their full compensation. The orbit alternately bulges out into a circle, and narrows into an ellipse. The earth is now, and has been for many ages, expanding into a circle, but it will again collapse into an ellipse. The orbit may be compared to a great heart beating the pulses of eternity, alternately expanding and contracting, and each systole and diastole occupying many thousands of years. The invariable length of the orbit, or rather of the major axis, has been justly styled the Magna Charta of the planetary system, as this is the basis of its stability. The oscillations in the breadth of the ellipse are styled the secular variations of the eccentricity, the eccentricity being the amount of departure from
