95 millions of miles; the time of its rotation, almost 24 hours; and of its revolution round the Sun 3654 days, moving at the rate of 68 thousand miles the hour. The inclination of the Earth's axis to its orbit is such, that the Sun appears directly over head at noon, to the extent of 23 degrees and 28 minutes on either side of the Equator. This occasions the variations in the length of days, and of the seasons; topics which will be resumed in a subsequent chapter.

That the Earth is a globe, is conclusively proved from each one of the following facts. A person at the Equator, would see the north polar star barely above the horizon, but as he travelled north, it would become more elevated till his arrival at the pole, when it would be immediately over his head. A traveller, jour* neying due east under the Equator, would have the Sun arrive at the meridian, one hour earlier for every 1040 miles over which he travelled. The earth has been frequently circumnavigated. A ship going out to sea, has its hull, or lowest part, disappear first, demonstrating that the ocean is convex, as seen at the bottom of Plate vii. The shadow of the Earth in eclipses of the moon, is always circular on every side. The other planets are known to be spherical. Matter, in the rain drop and in the world, tends to this form.

The connection between the rotation of the Earth on its axis, and its spheroidal form, or greater equatorial, than polar diameter, will be discussed in Physical Astronomy.

THE MOON.

THE Earth has one satellite, the Moon. Its diameter is about 2180 miles, its density, like its primary; its mean apparent diameter 312 minutes;

-its distance from the Sun ?-the time of its rotation?-of its revolution?-and its hourly rate? What distance from the Equator is the Sun ever vertical? For what will this account?

By what 7 arguments can it be demonstrated, that the Earth is a globe? What is the 1st ?-the 2d ?-the 3d ?-the 4th ? the 5th ?-the 6th ?-the 7th?

What is the magnitude of the moon? What is its density?

and its mean distance from the Earth about 240 thousand miles. }

It turns once on its axis in 27 days; the period in which it revolves once round the Earth, in a direction from west to east, and once a year it accompanies its primary round the Sun. The Moon is a dark body, shining only by reflection. This is proved, from its varying phases, and its eclipses; topics to be resumed in a future chapter.

Light is unequally reflected from different parts of the moon's disc, as viewed by the unaided eye. When examined by a telescope of high magnifying powers, the cause of this inequality becomes obvious. The surface of the moon is highly diversified by mountains, precipices and plains. The location of hundreds of these, has been ascertained, and names assigned to them. As the moon always presents nearly the same side to the Earth, and has an atmosphere only 1 miles high, the mountains and cavities can be measured. See a beautiful and accurate telescopic map, Plate i. Figure 5.

Schrocter, makes the highest mountain more than five miles high; and the deepest cavity more than 3 miles deep, and 40 miles in diameter. Herschel makes them less. The more even parts, or plains, are the darkest. Though some of them have the names of seas given to them, it is doubtful whether seas, or even large lakes, exist there. Water may exist in small quantities. Some of these plains have conical hills rising in them, with circular cavities near the summits. Many of the highest mountains appear to have been volcanoes, and a few are considered such at present. Dr. Herschel saw three which What its apparent diameter? What is its distance from the Earth? How is it found to be an opaque body? What occasions the inequality of light reflected from its surface? What is the height of its atmosphere ?

What is Schroeter's estimate of the height of mountains and depth of cavities on the surface of the Moon? What other facts can you state?

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he considered volcanic, and one of them in a state of eruption. The scenery there must be very romantic, and some of the rocks and precipices, tremendously terrific.

CHAPTER IV.

MARS.

NEXT without the orbit of the Earth, is the orbit of Mars. Hence it is called a superior planet. Its diameter is 4444 miles, its density 3 times the density of water, and its distance from the Sun, 144 millions of miles.} It turns on its axis in 24 hours,} and performs a revolution round the Sun in 687 days, or less than 2 years. Its torrid zone, extends about 28 degrees each side of its Equator. It moves above 50 thousand miles the hour, and. enjoys about half as much light from the Sun, as the Earth.)

Mars has an uncommonly red appearance, which is occasioned by the great density of its atmosphere. This accounts for the mountains not being visible, as in the planets Venus and Mercury. But the accumulation of whiteness at the pole, from which the Sun's rays are withdrawn, is indicative of vicis. situdes in seasons, similar to those on the Earth. See telescopic appearance of Mars, Plate i. Figure 3. Mars, in conjunction with the Sun, is 239 millions of files distant when in opposition only 49 millions of miles. Of course, it will vary from 9 seconds to 29 seconds, in its apparent diameter. The angle of its orbit with the Ecliptic is about 2 degrees.

Why is Mars called a superior planet? What is its magnitude? -its density? its distance from the Sun ?-the period of its rotation? and its hourly rate? What is the extent of its torrid zone, and its comparative amount of light? What is the color of Mars? and what its cause? and what its consequences? What variations are there in its polar appearances ?—in its distance? -and in its apparent magnitude