Page:Popular Science Monthly Volume 35.djvu/52

42 more than half of the mean diameter of the Earth. The density of the planet is rather less than three quarters of the density of the Earth, or about four times the density of water. The force of gravity upon its surface is less than two fifths of that upon the Earth; more accurately, 0⋅38. That is to say, if a man from the Earth could visit Mars, he would find that his weight had diminished almost two thirds. Members of terrestrial fat men's clubs could become agile dancers by simply going to Mars. This feebleness of the force of gravity must, it is clear, have an important effect upon the organization of any forms of life that may exist upon Mars, whether animal or vegetable. The mean distance of Mars from the sun is 141,500,000 miles, that of the Earth being 92,900,000. The length of Mars's year is six hundred and eighty-seven days. Its day is only forty-one minutes longer than our day upon the Earth. The inclination of its equator to the plane of its orbit differs but slightly from that of the Earth. But when we come to consider the eccentricity of its orbit, we find a decided difference between the Earth and Mars. The Earth's orbit is so nearly a circle that its greatest and least distances from the sun differ by only 3,000,000 miles, while the orbit of Mars is so eccentric that that planet is 26,000,000 miles nearer to the sun at one extremity of its orbit than at the other. It follows that, while Mars receives, upon the whole, less than half as much light and heat from the sun as the Earth gets, yet that quantity is variable to the extent of about one third of its greatest value—in other words, the sun gives Mars half as much again heat at its perihelion as it does at its aphelion. It is hardly necessary to point out the important climatic effect of such a variation. Another remarkable resemblance between the Earth and Mars comes in here. Just as on the Earth, the summer of the northern hemisphere of Mars occurs when the planet is farthest from the sun and its winter when nearest. The effect, as Mr. Proctor has pointed out, tends to equalize the temperature of the seasons in Mars's northern hemisphere, but to exaggerate their difference in the southern hemisphere.

We may dwell for a moment upon this last-stated peculiarity, for it is exceedingly interesting in its suggestiveness. Having summer occurring in the southern hemisphere of Mars at the planet's perihelion, and winter at its aphelion, we should find there a most remarkable disparity both in temperature and in the brilliancy of daylight between the two seasons. The difference would be the sum of the effects produced by the greater or less distance of the sun and the variation in the inclination of its rays to the surface of the planet. Since the first cause alone would produce an inequality amounting, in the extreme, nearly to the ratio of 3 to 2, it is evident that the addition of the second would