THE EYE.

THAT which is called the eye-ball is a hollow globe, filled with certain transparent humours, through which the light passes to the inside, or back part of the eye, called the

retina.

The light that is reflected from any thing we look at, conveys the image of that object to the retina; and this image or likeness is transmitted to the brain through the medium of the optic nerves.

The white of the eye contains a transparent substance, called the cornea; behind which is a partition called the iris, because of the various colours which it exhibits. It is from the apparent colour of the iris, that the eyes are said to be black, brown, gray, blue, &c.

In the centre of the iris there is an opening, called the pupil of the eye. This can be enlarged or diminished, so as to admit more or less light through it to the retina. Cats, owls, and many other nocturnal animals have very large pupils, which they nearly close in the day-time, so as to appear like a narrow streak; but at night or in the dark, they open the pupils wide, in order to admit the light and gather the scattered rays.

When there is too bright a glare of light, we naturally close or contract the pupil, so as to prevent its giving us pain. For the same reason, we cannot look at the sun, or any very bright dazzling object, without hurting the eye. But when we go from a light room, out in the air of a dark evening, at first it seems very dark: but in a

SPELL eye-ball; certain; transparent; define it. Spell humours; through; retina; reflected; define it. Spell conveys; image; transmitted; define it. Spell medium; optic; define it. Spell substance; transparent; cornea; iris; partition; colours; exhibits; centre; iris; pupil; diminished; define it. Spell nocturnal; define it. Spell streak; scattered naturally; dazzling; gradually; delicate; surrounded

little while it appears lighter, because we gradually enlarge the pupils of our eyes

The tender and delicate eye, being placed in a hollow, or socket surrounded with bones, is also protected from injury by means of the eye-lids and the eye-brows.

By the muscles of the eye-brows, and the strong hairs that grow on them, the eyes are shaded and often shielded from what would injure them.

But the wisdom and goodness of the Creator, are more abundantly evident in the protection and uses of the eyelids. These are furnished with delicate hairs called eyelashes, which form a kind of grate or screen in front of the eye, to defend it from outward dangers, and prevent the entrance of things that would hinder our sight.

The eye-lids also aid in preventing too much light from entering the eye, both when we are awake and when we sleep. But if any dust, or motes, or small particles enter the eye, by the act of winking, they assist in wiping them out.

In order to keep the eye moist and clean, so that it may be bright and clear for seeing objects, a wash or liquid is constantly supplied for that purpose, and spread over the ball of the eye by the action of the lids in winking.

This wash or moisture is the same as tears; only it is furnished in very small portions, in common. It proceeds from a little spongy, fleshy substance, called a gland, which lies under the bone on which the eye-brow is placed, and is out of sight on the side next the temples.

Every time we wink or shut our eyes, these tear-glands are pressed on by the upper eye-lids, and a little moisture is forced out and spread over the ball of the eye, in order to wash it and keep it clean.

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All the dust and fine particles that may get into the eye, are thus washed off by this tear-water, and carried to the corner of the eye next the nose. Here are two little canals in each eye, which convey the tears into larger tubes that pass through the bone, and empty them into the nose; while the dust is sometimes left in the corner of the eye in a little ball, as it is often found in the morning when w awake.

If through joy, or grief, or fear, or pain, or by reason of any thing getting into the eye that causes pain, there is a greater portion of this tear-water pressed out of the glands. than can find vent through the tubes into the nose, the eyes overflow, and the tears run down the cheeks. In persons of weak eyes, the same effect is sometimes produced by going out in a cold wind, or looking too intensely at bright or small objects.

If the tubes conveying the water from the eyes into the nose should become stopped, diseases in the eyes might ensue, and the sense of smelling become impaired.

LATITUDE, LONGITUDE, LENGTH OF DAYS, &c. THE earth on which we live is a large globe, nearly eight thousand miles in diameter; and it turns round on its axis once in twenty-four hours.

The axis of the earth is a line which is supposed to pass through the middle of it from north to south; the north end of which is called the north pole, and the south end thereof, the south pole.

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SPELL earth; eight; thousand; diameter; axis; through; skies; continued; equally; equator; surface; latitude; reckoned; circle;

If the north end of this axis were continued to the skies, it would strike the north star, very nearly. Hence, the north star is often called the pole-star.

The middle of the earth, or a line drawn round it from east to west, and every where equally distant from the poles, is called the equator.

The distance of any place on the surface of the earth from the equator, is called the latitude of that place.

Latitude is reckoned in degrees and minutes; because every circle is supposed to be divided into three hundred and sixty degrees, and every degree into sixty minutes.

The poles are every where ninety degrees from the equa tor; and as the latitude is reckoned from the equator towards the poles, so it is evident that latitude can only be counted in a quarter of a circle; that is, within ninety degrees, and it is either north or south.

As every degree on the equator is found to be about sixtynine miles and a half, so the latitude of any place, in miles, is so many times sixty-nine and a half miles, as there are degrees in the latitude.

Philadelphia is in about forty degrees of north latitude. If we multiply forty by sixty-nine and a half, we shall have two thousand seven hundred and eighty; the number of miles from the equator to Philadelphia.

In geography, every place has its latitude and its longitude.

Longitude is the distance, in degrees and minutes, reckoned on the equator from some meridian fixed on as a standard, and it is either east or west.

In maps, the degrees of latitude are commonly placed at the sides or borders of the map, and are north latitude, if

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above the equator, and south, if below it. The degrees of longitude are placed at the top and bottom of the map, and are either east or west.

In globes that represent the earth, the meridian lines on which latitude is reckoned, are circles passing through the equator from north to south, at right angles. But every place between these circles is supposed to have its own meridian, or north and south line.

Longitude is marked in degrees, and is reckoned on the equator only.

When the sun appears to be on, or directly over any meridian, it is noon, or the middle of the day, to all the places that are on that meridian circle.

The line or circle which bounds our view all around us, is called the horizon.

As the earth turns on its axis, and all the countries and people on its surface turn with it, so the sun, moon, and stars appear to rise and set in the horizon of every place.

But the length of the day is different in different places on the globe; because they are nearer to or farther from the equator, where the days are always twelve hours long.

The earth goes round the sun once every year in a great ckcle or ellipsis, called the earth's orbit, which is about one hundred and ninety millions of miles in diameter. But the equator, or middle of the earth is not always in the line of this orbit circle.

The part of the earth which is in the line or plane of this orbit, varies from the equator, and is one-half of the year north of it, and the other half south. The line or part of the earth which is always toward the sun in this orbit circle, is called the ecliptic line.

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