But this general movement, as we have seen, is, in itself, simply the mean of a series of oscillations, which D'Alembert has also connected with gravitation. He has shown that the nutation of the earth's axis results from the moon's attraction on the bulging portion of our globe. Finally, it has been mathematically demonstrated that the said bulging portion of the earth produces, under the continuous action of the sun, the precession of the equinoxes; just as this portion determines, by its continuous action, the nutation of the lunar axis. As in this universal ponderation all the wheelwork of the world catches (_tous les rouages du monde s'engrènent_), and the planets, such as Mars and Venus, must also have their share in the action, however weak it may be, we have contrived to render an exact account of the slow changes of the obliquity of the Ecliptic. Let us resume. Movement and matter, all is _ponderated_. Inasmuch as matter is unequally distributed around the earth's centre, being flattened at the Poles and bulging at the Equator, it follows that the sun's enormous weight makes it vacillate, so that it describes at its axis a cone around the poles of the plane of its orbit. Its movement we see in the heavens in the precession of the equinoxes. But the terrestrial axis traces it tremblingly, because the moon, owing to its vicinity, exercises a perturbing action on our planet, which, in its turn, exercises on the moon a still more energetic influence. [Illustration] FOOTNOTES: [Footnote 38: Seneca, "Quæstiones Naturales," vi. 6.] [Footnote 39: Lactantius, "De Falsa Sapientia," iii. 24.] [Illustration] CHAPTER II. _WHAT MAY BE SEEN UPON THE EARTH._ "There lives and works A soul in all things, and that soul is God." --COWPER. [Illustration] We have returned, at least in an astronomical sense, to the budding, happy, radiant spring; the sun, in its apparent course, crosses the equinoctial line; the duration of the day, transiently equal to that of the night, will augment in proportion as the great luminary describes above our horizon greater and yet greater arcs of a circle. Yet this is not the budding, happy, radiant spring of the poets. No, if it be spring according to the law of universal gravitation, it is winter still by the law of life. The forest trees, such as the oak, the ash, the fir, and the beech, continue to present the image of death; and the sap which should reanimate them has not awakened from its winter sleep. A solemn moment is it when the sap--that life-blood of the plant--arrested by the icy grasp of winter in its circulatory movement--receives a new impulse through the vivifying action of the central luminary of our system. What a subject for study and reflection! [Illustration: FIG. 28.--Landscape in Winter.] It has been very finely dealt with by Longfellow.