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while we sit still and watch them pass. On a 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 down stream. "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 as 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 Eevolution 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; but 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 its 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 half its surface only illuminated at one time by the sun. To a person at D, the sun is in the horizon and day commences, the luminary appearing to rise higher and higher in the heavens with a westerly motion, as the observer is carried forward easterly by the earth's diurnal rotation to A, where he has the sun in his

Fig. 28.

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meridian, and it is consequently noon. The sun then begins to decline in the sky until the spectator arrives at B, where it sets, or is again in the horizon on the west side, and night begins. He moves on to C, which marks his position at midnight, the sun being then on the meridian of places on the opposite part of the earth, and he is then brought round again to D, the point of sunrise, when another day commences. (Hind.)

'The unequal rate of diurnal motion.—Different points upon the surface of the earth revolve with different velocities. At the two poles the speed of rotation is nothing, while at the equator it is greatest, or over 1,000 miles per hour. At Quito, the circle of latitude is much longer than one at the mouth of the St. Lawrence, and the velocities vary in the same proportion. The former place moves at the rate of about 1,038 miles per hour; the latter, 450 miles. In our latitude (41°) the speed is about 780 miles per hour. We do not perceive this wonderful velocity with which we are flying through the air, because the air moves with us.* Yet were the earth suddenly to stop its rotation, the terrible shock would, without doubt, destroy the entire race of man, and we, with houses, trees, rocks, and even the oceans, in one confused mass, would be hurled headlong into space. On the other hand, were the rate of rotation to increase, the length of the day would be proportionately shortened, and the weight of all bodies decreased by the centrifugal force thus produced. Indeed, if the rotary movement should become swift enough to

* An ingenious inventor once suggested that we should utilize the earth's rotation, as the most simple and economical, as well as rapid mode of locomotion that could be conceived. This was to be accomplished by rising in a balloon to a height inaccessible to aerial currents. The balloon, remaining immovable in this calm region, would simply await the moment when the earth, rotating underneath, would present the place of destination to the eyes of travellers, who would then descend. A wellregulated watch and an exact knowledge of longitudes would thus render travelling possible from east to west, all voyages north or south naturally being interdicted. This suggestion has only one fault; it supposes that the atmospheric strata do not revolve with the earth. Upon that hypothesis, since we rotate in our latitude with the velocity of 333 yards in a second, there would result a wind in the contrary direction ten times more violent than the most terrible hurricane. Is not the absence of Buch a state of things a convincing proof of the participation of the atmospheric envelope in the general movement? (Guillemin.)

reduce the day to 84 minutes, or about TV its present length, the force of gravity would be overcome, and, at the equator, all bodies would be without weight; if the speed were still further increased, loose bodies would fly off from the earth like water from a grindstone when swiftly turned, while we should be compelled ta constantly "hold on" to avoid sharing the same fate. But against such a catastrophe we are assured by the immutability of God's laws. "He is the same yesterday, to-day, and forever." The earth has not varied in its revolution of a second in 2,000 years.

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Unequal diurnal orbits of the stars.—Let O represent our position on the earth's surface, E Z B our meridian; EIB K our horizon; P and P' the north

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