We know that the sole cause of the inequality of the seasons, as well as of day and night in the planets, is the inclination of the star on its axis of rotation. If the planets, while revolving round the sun, retained the verticality of the axis which joins these north and south poles, there would be perfect equality in the distribution of the solar light and heat over the same latitudes; along each parallel there would be a complete regularity in the lighting and warming of the planet; the differences of heat and cold would not depend on anything but their greater or less distance from the sun. But this verticality only exists for two or three planets of our system. The others, and among them Mercury, Venus, the Earth, and Mars, are strongly inclined on their axis of rotation. They revolve in a bent position, as if they had received a great blow on the shoulder, which had caused them to deviate from their primitive and regular situation. From this there results a very variable disposition of the duration of the light, and consequently of the heat, which these inclined planets receive from the horizontal rays of the solar star. Thus the inequality in the length of the days and nights, and the diversity of the four seasons on the same parallel, are accounted for.[17] [Illustration: Mercury. Venus. Earth. Mars. Sun. Fig. 4.--Comparative Size of the Planets Mercury, Venus, the Earth, and Mars.] The inclination of the axis of the terrestrial sphere is 23° which is a considerable deviation, and occasions great differences in the duration of days and of seasons on different points of our globe. The inclination of the axis of the planet Mercury is enormous: it is 70°. This planet bends over itself as if about to fall. Hence results prodigious variation of light and heat on the same parallel, and seasons whose abrupt changes must be painful and hard to bear by the inhabitants of this planet, if such inhabitants exist. Mercury is five times less than the Earth, as is shown in plate 4. Venus comes after Mercury, according to distance from the Sun. Venus, which is 27,000,000 of leagues from the Sun, receives twice as much light and heat as our globe. Its days are of nearly the same length as ours (23 hours, 21 minutes), but its year, necessarily shorter than that of the Earth, since it is nearer to the Sun, lasts only 224 days. Its seasons last two months each. Its globe is nearly of the same bulk as that of the Earth. Venus is almost always wrapped in clouds, which must fall in rain, forming rivers and seas. These waters refresh the plains, which must be scorched by the heat of the burning sun. The seasons are still shorter and more unequal in Venus than in Mercury; its axis is, in fact, inclined at 75°.