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clopedia, the equatorial diameter is reckoned at 7977, the polar at 7940, considered by the author but "an approximation to a true estimation." In the Practical Navigator of Dr. Bowditch, the diameter is considered 7964. Thus the mean diameter will be considered in this compend.

The errors of antiquity, of childhood, and ignorance, in considering the Earth an extended plain, or unbounded in its dimensions, are corrected by philosophy. Its true form is now well known to the scientific world. But the astronomical student is in danger of verging to the opposite extreme. When he considers the Earth as a planet, greatly inferior in magnitude to several wandering orbs of his own solar system; immensely less than the Sun; and the Sun but a speck in the Creator's works, he seems to contract its true dimensions, and to be insensible, that still, to its inhabitants, it is a globe of vast magnitude; of which and its kindred orbs it may be truly said, "these little things are great to little man." Considering the diameter of the Earth 7964 miles, the circumference is about 25,020 miles, and the superficial contents, or surface, 199,259,280 square miles.

If a person were so elevated that he could see twenty miles in every direction, his prospect would include a region of more than 1256 square miles. Should he survey a new region of this extent every day, he would not pass over the whole of the Earth's surface under 434 years. Thus the life of an antediluvian would be necessary for such a survey.

Were a being of sufficient longevity and suitably constituted to undertake an examination of the Earth's surface by traversing every square mile, and were he to proceed with such rapidity as to inspect a mile an hour, or twelve miles in the light of every day, he would find himself far short of his purpose at the end of 45,000 years.

Six great circles are conceived drawn round the Earth; the equator, ecliptic, meridian, horizon, and two colures.

The equator is an imaginary circle encompassing the Earth from east to west; the plane of the circle dividing it into northern and southern hemispheres.

The ecliptic is a great circle, in which the Earth performs its annual revolution; or in which the Sun appears to perform an annual revolution round the Earth. It is divided into twelve equal parts, denominated the twelve signs of the ecliptic, each containing 30°; Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricornus, Aquarius, Pisces.

The plane of the equator is inclined to the ecliptic in an angle of about 23° 28'. (See Obliquity.)

It

The meridian is a great circle encompassing the Earth from north to south, passing through the poles, and crossing the equator at right angles. It is represented on an artificial globe by a graduated circle of brass. is called meridian, from the Latin word meridies, midday, because, when it arrives at the Sun, the time is noon, or the middle of the day.* As many meridians may be conceived as there are places east or west of each other; but all places directly north or south of each other have the same meridian.

The horizon is a great circle surrounding the Earth, ninety degrees distant from the zenith and nadir of any place. The plane of this circle, passing through the centre of the Earth, divides it into upper and lower hemispheres. This is called the real or rational horizon. The sensible horizon is the circle which limits our view, dividing the visible part of the heavens from the invisible. The horizon is represented on an artificial globe by a circle of wood having several other circles drawn upon its surface.

The colures are meridians; but meridians particularly distinguished. The equinoctial colure is a line drawn round the Earth from north to south through the equinoctial points Aries and Libra. The solstitial colure is a line drawn round the Earth in the same di

* Half of this circle is usually called a meridian.

rection; but through the solstitial points Cancer and Capricorn.

There are four less circles represented on an artificial globe, and considered circles of the Earth; two tropics and two polar circles. They are drawn parallel to the equator. The northern tropic, called the tropic of Cancer, encompasses the Earth at about 23° 28′ north of the equator; the southern, called the tropic of Capricorn, at the same distance south of the equator.

The polar circles are drawn round the Earth at about 23° 287 from the poles. The northern is called the Arctic, the southern the Antarctic circle.

Latitude is the distance from the equator north or south. It is reckoned in degrees,* minutes, and seconds. Lines of latitude are drawn on a terrestrial globe, on each side of the equator, and parallel to that circle. They are called parallels of latitude. These are considered circles of the Earth, and are at stated distances from the equator to the poles. They may, however, be conceived to be drawn at any distance from each other, or from the equinoctial, every place, north or south, having its own parallel.

Longitude is the distance east or west from some fixed meridian. This also is reckoned in degrees and sexagesimal parts of a degree. It increases each way from the meridian; 180°, or half way round the globe, being the highest longitude. (See article Longitude.)

The Earth is embraced by five zones; the torrid, two temperate, and two frigid. The torrid zone (Plate v. Fig. 6,) extends from the equator each way to the tropics; the temperate zones include the whole space from the tropics to the polar circles; and the frigid zones the remaining space from the polar circles to the poles.

The division of the Earth's surface into zones is not

* Every circle, large or small, is divided into 360°. A degree on the surface of the Earth is the 360th part of its circumference. The centre of this circle must be at the centre of the Earth. See a terrestrial artificial globe.

The torrid

imaginary, but has a foundation in nature. zone comprehends all that region where the Sun is vertical at any season of the year. The temperate zones spread over the whole of the Earth's surface from the tropics to the extreme limit of continual and successive day and night, the Arctic and the Antarctic circles being drawn at the bound, where the longest day is 24 hours. At that bound the Sun does not appear to set at the summer solstice, nor to rise at the winter solstice.

The frigid zones are enveloped in light and darkness in alternate succession. The Sun, at its greatest declination north, shines over the north pole to the Arctic circle. The whole northern frigid zone is then illuminated, and, by the diurnal motion of the Earth, revolves wholly in the light. The southern frigid zone, precluded from the Sun's rays, is then involved in entire darkness. When the Sun is in his greatest declination south, shining over the south pole to the Antarctic circle, the southern frigid zone is enlightened; the northern, abandoned by the Sun, is shrouded in darkness. The continuance of light or darkness in the Arctic and Antarctic regions is longer, the nearer any place is to either pole, where the day and the night continue alternately for six months; except the greater prevalence of light from refraction and other causes.

The Earth has three motions; its diurnal rotation on its axis; its annual motion in its orbit round the Sun; and the revolution of its axis round the poles of the ecliptic.

The rotation of the Earth on its axis is performed in 23 h. 56 m. 4 s. or one sidereal day. This is a most uniform motion. By bringing the different parts of the Earth to the Sun in succession, it produces day and night. Given to this Earth, at its creation, by an all-benevolent Creator, it continues a constant return of blessings to his dependent creatures. This motion is from west to east. It causes the apparent revolution of the heavenly bodies in a contrary direction, from east to

west.

Different parts of the Earth, in this rotation, move with unequal velocity. Greatest at the equator, it decreases towards the poles, as the cosines of the latitude decrease. A place in Borneo or the Colombian republic, at the equator, moves about 1042 miles an hour; Washington city, 811 miles; Boston, 770 miles; London, 649 miles; St. Petersburg, 522 miles; an inhabitant of Greenland, in latitude 800, only 181 miles. When this motion is on the side of the Earth opposite the Sun, it nearly coincides with the immense velocity of the Earth in its orbit. By this motion the centrifugal force of an object near the equator is greater than at any parallel of latitude. This, as well as its distance from the centre of gravity, causes objects to be lighter at the equator than near the poles. The farmer and mechanic know, that the water on a grindstone, turned swiftly round, rises towards the highest part, and flies off by increased velocity. A similar effect would be produced on the Earth, were the motion sufficiently increased. "If," says Dr. Enfield, "the diurnal motion of the Earth round its axis was about 17 times faster than it is, the centrifugal force would, at the equator, be equal to the power of gravity, and all bodies there would entirely lose their weight. But if the Earth revolved still quicker than this, they would all fly off.”

The circles, which the heavenly bodies appear to describe by this motion of the Earth on its axis, assume a different position as seen from different parts of the Earth's surface; the great concave of the heavens, or celestial sphere, changing its appearance, as differently viewed by the spectator.

At the equator, the inhabitants have a right sphere, all the heavenly bodies appearing to rise and set at right angles to the horizon. The celestial equator passes through the zenith and nadir. The poles are in the horizon.

From the equator to the poles, the inhabitants have an oblique sphere. The apparent circles, or circles formed by the apparent motion of the heavenly bodies, are

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