But, owing to her extensive atmosphere, there would be a very broad band of twilight on Venus, running entirely around the planet at the inner edge of the light hemisphere. What the meteorological conditions within this zone would be is purely a matter of conjecture. As in the case of Mercury, we should expect an interchange of atmospheric currents between the light and dark sides of the planet, the heated air rising under the influence of the unsetting sun in one hemisphere, and being replaced by an indraught of cold air from the other. The twilight band would probably be the scene of atmospheric conflicts and storms, and of immense precipitation, if there were oceans on the light hemisphere to charge the air with moisture. It has been suggested that ice and snow might be piled in a vast circle of glaciers, belting the planet along the line between perpetual day and night, and that where the sunbeams touched these icy deposits near the edge of the light hemisphere a marvelous spectacle of prismatic hills of crystal would be presented! It may be remarked that it would be the inhabitants of the dark hemisphere who would enjoy the beautiful scene of the earth and the moon in opposition. CHAPTER IV MARS, A WORLD MORE ADVANCED THAN OURS Mars is the fourth planet in the order of distance from the sun, and the outermost member of the terrestrial group. Its mean distance is 141,500,000 miles, variable, through the eccentricity of its orbit, to the extent of about 13,000,000 miles. It will be observed that this is only a million miles less than the variation in Mercury's distance from the sun, from which, in a previous chapter, were deduced most momentous consequences; but, in the case of Mars, the ratio of the variation to the mean distance is far smaller than with Mercury, so that the effect upon the temperature of the planet is relatively insignificant. Mars gets a little less than half as much solar light and heat as the earth receives, its situation in this respect being just the opposite to that of Venus. Its period of orbital revolution, or the length of its year, is 687 of our days. The diameter of Mars is 4,200 miles, and its density is 73 per cent of the earth's density. Gravity on its surface is only 38 per cent of terrestrial gravity--i.e., a one hundred-pound weight removed from the earth to Mars would there weigh but thirty-eight pounds. Mars evidently has an atmosphere, the details of which we shall discuss later. The poles of the planet are inclined from a perpendicular to the plane of its orbit at very nearly the same angle as that of the earth's poles, viz., 24 deg. 50 min. Its rotation on its axis is also effected in almost the same period as the earth's, viz., 24 hours, 37 minutes.