pearing to rise higher and higher, 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 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 brought round again to D, the point of sunrise, when another day commences.

UNEQUAL RATE OF DIURNAL MOTION.-Different points upon the surface of the earth revolve with different velocities. At the 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 the 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, 682 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 ether, because the atmosphere moves with us.*

Were the earth suddenly to stop its rotation, the terrible shock would, without doubt, destroy the

"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, should present the place of destination to the eyes of travelers who would then descend. A well-regulated watch and an exact knowledge of longitude would thus render traveling possible from

entire race of man; while we, with houses, trees, rocks, and even the oceans, would be hurled, in one confused mass, 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. If the rotary movement should become swift enough to reduce the day to eighty-four minutes, 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 swiftly-turned grindstone, while we should be compelled constantly to "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."

UNEQUAL DIURNAL ORBITS OF THE STARS.-In figure 35, let O represent our position on the earth's surface; E Z B, our meridian; E I B K, our horizon; P and P', the north and south poles; Z, the zenith;

east to west, all voyages north or south 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 (at London) 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 such a state of things a convincing proof of the participation of the atmospheric envelope in the general movement?"GUILLEMIN.

*Laplace concluded in 1799 that the inequalities of the earth's rotation were too insignificant for measurement. But, more recently. Delaunay has shown from the moon's acceleration that a minute change, caused by the friction of the sea and atmosphere upon the earth's surface, has taken place, producing a variation in the length of the day. The acceleration of the moon in its path, is, however, only seven feet per century, or less than an inch per annum, and the time of the earth's rotation has increased but of a second in 2,400 years.-BALL.

Z', the nadir; and GICK the celestial equator. Now P B, it will be seen, is the elevation of the north pole above the horizon, or the latitude of the place.

Suppose we should see a star at A, on the meridian below the pole. The earth revolves in the direction GIC; the star will therefore move along A L to Z, when it is on the meridian above the pole. It continues its course along the dotted line around to A again, when it is on the meridian below the pole, having made a complete circuit around the pole, but not having descended below our horizon.

A star rising at B would just touch the horizon; one at I would move on the celestial equator, and would be above the horizon as long a time as it is below,―twelve hours in each case; a star rising at M would come just above the horizon and set again at N.

UNEQUAL DIURNAL VELOCITIES OF THE STARS.The stars appear to us to be set in a concave shell which revolves daily about the earth. As different parts of the earth really rotate with varying velocities, so the stars appear to revolve at different rates of speed. Those near the pole, having a small orbit, revolve very slowly, while those near the celestial equator move at the greatest speed.

APPEARANCE OF THE STARS AT DIFFERENT PLACES ON THE EARTH.-Were we placed at the north pole, Polaris would be directly overhead, and the stars would seem to pass around us in circles parallel to the horizon, and increasing in diameter from the upper to the lower ones. Were we placed at the equator, the pole-star would be at the horizon, and the stars would move in circles perpendicular to the horizon, and decreasing in diameter, north and south from those in the zenith, while we could see one half of the path of each star. Were we placed in the southern hemisphere, the circumpolar stars would revolve about the south pole, and the others in circles resembling those in our sky, only the points of direction would be reversed to correspond with the pole. Were we placed at the south pole, the appearance would be the same as at the north pole, except that no star is there to mark the direction of the earth's axis.

Motion of the Earth in Space about the Sun.The earth revolves in an elliptical path about the sun at a mean distance of 93,000,000 miles.

The eccentricity of this path, which is greater than that of the orbit of Venus, changes about

100 per century. The orbit would, therefore, finally become circular, were it not that, after the lapse of some thousands of years, the eccentricity will begin to increase again, and will thus vary through all time within definite, although yet undetermined limits. The circumference is nearly 600,000,000 miles, and the earth pursues this wonderful journey at the rate of over eighteen miles per second.

This revolution of the earth about the sun gives rise to various phenomena, of which we shall now proceed to speak.

1. CHANGE IN THE APPEARANCE OF THE HEAVENS IN DIFFERENT MONTHS.-In Fig. 36, suppose A B C D to be the orbit of the earth, and EFGH the sphere of the fixed stars, surrounding the sun in every direction. When our globe is at A, the stars about E are on the meridian at midnight. Being seen from the earth in the quarter opposite to the sun, they are favorably placed for observation. The stars at G, on the contrary, will be invisible, for the sun intervenes between them and the earth they are on the meridian of the spectator about the same time as the sun, and are hidden in his rays.

In three months, the earth has passed over onefourth of its orbit, and has arrived at B. Stars about F now appear on the meridian at midnight; those at E, which previously occupied their places, have descended toward the west; while those about G are just coming into sight in the east.

In three months more, the earth is situated at C,