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in Appendix.) If we represent the earth by a globe one yard in diameter, the polar diameter would be one-tenth of an inch too long. It has been recently

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shown that the equator itself is not a perfect circle,

but is somewhat flattened, since the diameter whicli pierces the meridian 14° east of Greenwich is two miles longer than the one at right angles to it. The circumference of the earth is about 25,000 miles. Its density is about 5£ times that of water. Its weight is

6,069,000,000,000,000,000,000 tons.

The inequalities of its surface, arising from buildings, valleys, mountains, etc., have been likened to the roughness on the rind of an orange. This is not an exaggeration. On a globe sixteen inches in diameter, the land, to be in proportion, should be represented by the thinnest writing-paper, the hills by small grains of sand, and elevated ranges by thick drawing-paper. To represent the deepest wells or mines, a scratch might be made that would be invisible except with a glass. The water in the ocean could be shown by a brush dipped in color and lightly drawn over the bed of the sea.

The Eotundity Of The Earth.—This is shown in various ways, among which are the following: (1) By the fact that vessels have sailed around the earth ;*

* It is curious, in connection with this well-known fact, to recall the arguments urged by the Spanish philosophers against the reasoning of Columbus, when he assured them that he could arrive at Asia just as certainly by sailing west as east. "How," they asked, "can the earth be round? If it were, then on the opposite side the rain would fall upward, trees would grow with their branches down, and everything would be topsy-turvy. Every object on its surface would certainly fall off and if a ship by sailing west should get aronnrf (2) when a ship is coming into port we see the masts first; (3) the shadow of the eartb on the moon is circular; (4) the polar star seems higher in the heavens as we pass north; (5) the horizon expands as we ascend an eminence.* If we climb to the top of a hill, we can see further than when on the plain at its foot. Our eyesight is not improved; it is only because ordinarily the curvature of the earth shuts off the view of distant objects, but when we ascend to a higher point, we can see farther over the side of the earth. The curvature is eight inches per mile, 22 x 8ln- = 32 inches for two miles, 31 x 8in for three miles, etc. An object of these respective heights would be just hidden at these distances.

Apparent And Real Motion.—In endeavoring to understand the various appearances of the heavenly bodies, it is well to remember how in daily life we transfer motion. On the cars, when in rapid movement, the fences and trees seem to glide by us,

there, it would never be able to climb up the side of the earth and get back again. How can a ship sail up hill?"

* The history of aeronautic adventure affords a curious illustration of this same principle. The late Mr. Sadler, the celebrated aeronaut, ascended on one occasion in a balloon from Dublin, and was wafted across the Irish Channel, when, on his approach to the Welsh coast, the balloon descended nearly to the surface of the sea. By this time the sun was set, and the shades of evening began to close in. He threw out nearly all his ballast, and suddenly sprang upward to a great height, and by so doing brought his horizon to dip below the sun, producing the whole phenomenon of a western sunrise. Subsequently descending in Wales, he, of course, witnessed a second sunset on the same evening.

while we sit still and watch them pass. On 11 bridge, when we are at rest, we follow the undulations of the waves, until at last we come to think that they are stationary and we are sweeping forward. "In the cabin of a large vessel going smoothly before the wind on still water, or drawn along a canal, not the smallest indication acquaints us with the 'way it is making.' We read, sit, walk, as if we were on land. If we throw a ball into the air, it falls back into our hand; if we drop it, it alights at our feet. Insects buzz around us as in the free air, and smoke ascends in the same manner as it would do in an apartment on shore. If, indeed, we come on deck, the case is in some respects different; the air, not being carried along with us, drifts away smoke and other light bodies such as feathers cast upon it, apparently in the opposite direction to that of the ship's progress; but in reality they remain at rest, and we leave them behind in the air."*

Diurnal Kevolution Of The Earth Around Its Axis.—The earth, in constantly turning from west

* '' And what is the earth itself but the good ship we are sailing in through the universe, bound round the sun; and as we sit here in one of the 'berths,' we are unconscious of there being any 'way' at all upon the vessel. On deck, too, out in the open air, it's all the same as long as we keep our eyes on the ship; bu'i immediately we look over the sides—and the horizon is but the 'gunwale' of our vessel—we see the blue tide of the great ocean around us go drifting by the ship, and sparkling with ite million stars as the waters of the sea itself sparkle at night between the tropics."

to east, elevates our horizon above the stars on the west, and depresses it below the stars on the east. As the horizon appears to us to be stationary, we assign the motion to the stars, thinking those on the west which it passes over and hides to have sunk below it or set, and imagining those on the east it has dropped below to have moved above it or risen. So, also, the horizon is depressed below the sun, and we call it sunrise; it is elevated above the sun, and we call it sunset. We thus see that the diurnal movement of the sun by day and stars by night is a mere optical delusion—that here as elsewhere we simply transfer motion. This seems easy enough for us to understand, because the explanation makes it so simple; but it was the "stone of stumbling" to ancient astronomers for two thousand years. Copernicus himself, it is said, first thought of the true solution while riding on a vessel and noticing how he insensibly transferred the movement of the ship to the objects on the shore. How much grander the beautiful simplicity of this theory than the cumbersome complexity of the old Ptolemaic belief!

Diurnal motion of the Sun.—The explanation just given illustrates the apparent motion of the sun, and the cause of day and night. Suppose S to be the sun. E, the earth, turning upon its axis EF from west to east, has only half its surface illuminated at one time by the sun. To a person at D, the sun is in the horizon and day commences,

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