The distance of Mercury from the Sun is commonly said to be about thirty-six millions of miles. That may be called his “middle-distance.” He draws sometimes as near to the Sun as twenty-eight millions of miles, and goes as far off as forty-three millions of miles--a very great difference. Even at his farthest Mercury has to endure an awful blaze of heat and glare; and at his nearest the most scorching mid-day ever known on the hottest parts of the Earth would be icy by comparison. Mercury does not always go at the same pace through the sky. When near he travels faster, because the Sun’s pull is stronger. When farther off he slackens his speed. At his quickest he whirls onward at the rate of about _thirty-five miles each second_! Think of that! A railway train does pretty well if it gets over about thirty-five miles of ground each _hour_, and sixty or seventy miles an hour we count very fast travelling. But Mercury’s speed is more than two thousand miles an hour. This quite puts our express trains to the blush. It is impossible for us to see much of a planet so bathed in sunlight glory. We do not know whether the axis of Mercury does or does not slant, like the Earth’s axis. Nor are we at all sure how long it takes Mercury to spin upon his axis. Wonderful as it sounds, Mercury has been weighed by man--and not only Mercury, but the Moon, and the Sun, and the other planets, and even some of the Stars. I cannot try, in a book such as this, to explain _how_ the heavenly bodies are weighed from our little Earth. I can only tell you that it is really and truly done. Mercury is a far heavier body than our Earth; not actually more heavy as a whole, because so much smaller, but heavier in _make_. Do you see what this means? Iron is heavier than tin in its make. A large quantity of tin may weigh more than a small lump of iron; yet in actual make the iron is heavier. If Mercury were as big as our Earth, Mercury would be very much the heavier of the two. Our Earth is less heavy in make than Mercury, but she comes next after Mercury. Other worlds are still lighter. Mercury shines less brightly than Venus, as seen from the Earth. We have to allow for the greater distance of Mercury; but even then Venus seems to be more brilliant than one would expect, while Mercury is less brilliant. Although the Sun pours his beams upon all things alike, those beams are not always received alike. Some worlds make more of the light which they have than do others, and they give out more shining in return. We see that even on the Earth. If a sheet of polished silver and a sheet of unpolished lead are held side by side in the sunlight, what a difference we find! The silver flashes brilliantly, while the lead shows only a dull sort of brightness. Of these two worlds, so near to the Sun, Mercury, the nearer, is said to shine only like lead, while Venus, the farther, shines like silver.