The elliptical form of the earth’s orbit and the unequal pace of the globe in the various points of its course cause some considerable variations in the duration of the seasons. In fact, from the 20th of March to the 22d of September, that is, during the spring and summer of the Northern Hemisphere, the earth takes 186 days to travel over the first and largest half of its orbit, while during the winter period, from the 22d of September to the 20th of March, only 179 days are required to accomplish the second half of its journey. The summer period of the Northern Hemisphere actually exceeds by seven or eight days, or about 187 hours, the corresponding period in the southern half of the globe; added to this, in consequence of the longer space of time during which the Arctic Pole remains inclined toward the sun in the regions north of the equator, the hours of daylight exceed the hours of night, while in the south the hours of darkness predominate. This is, however, to some extent compensated for; as, although in the southern regions of the earth the summer lasts a shorter time, our planet is then closer to the sun; it is at its perihelion, and consequently receives a larger proportion of heat. There is, however, no doubt about the fact—as it is proved by a direct observation, both of the winds and currents, and also of their various temperatures—that, taking an equal distance from the equator, the southern regions are colder than those of the north. If an equality of seasons between the two halves of the world does not at present exist, it will not fail to be established after a long series of centuries by means of a slow terrestrial movement, which has been known by the name of the _precession of the equinoxes_. Just as a top (if we may be allowed to avail ourselves of so old an illustration) turns round on the ground and bends over successively in every direction, thus describing with its axis an ideal cone, so the earth revolves in space, and slowly sways the line of its poles. This line, which is always sloped at an angle of 66° 32′ to the plane of the terrestrial orbit, turns round with a slight lateral motion, so as always to point to a new region of the sky; if it were prolonged indefinitely it would describe a circle amid the distant stars. As the axis of the earth is constantly changing its direction in this way, the plane of the equator must vary exactly to the same extent in its position as regards the sun. In fact, every year the exact moment of the March equinox anticipates by about twenty minutes the time at which the corresponding equinox fell in the year preceding. Each revolution of the earth round the sun brings a fresh advance of twenty minutes in the determination of the equinox; and as, during the long course of ages, the axis of the earth does not intermit in this swaying motion, the time must come, after a period of 12,900 years, that the conditions of the seasons will be altogether changed.