(b) In order of distance from the sun: Mercury, Venus, the Earth, Mars, Jupiter Saturn, Uranus, Neptune.

All the planets, except Uranus and Neptune, are visible from the earth to the unaided The earth, viewed from any of the other planets, would appear as a more or less brilliant "star."

eye.

Form and size of the Earth: The earth is very nearly a globe in shape, and measures about 25,000 miles round; while a straight line passing through its centre would be nearly 8000 miles long. That is, in round numbers,' the earth's circumference is 25,000 miles, and its diameter, 8000 miles. "

Proofs of the Earth's Rotundity: That the earth is round or globular in shape is proved by

(1) Its circumnavigation: By sailing constantly in the same general direction, westward or eastward, ships return to the port from which they set out. A person starting from Liverpool on a journey round the world would go by steamer to New York, and thence by rail to San Francisco. Then by steamer across the Pacific to Hong Kong, and home via Singapore and the Suez Canal. Trace the route on a globe, or on a map of the World on Mercator's Projection, and you will see that the traveller's course is throughout to the west. The earth therefore must be round or globular, otherwise it could not be circumnavigated.

(2) The shadow which the earth casts on the moon during an eclipse is always circu ar and as round or spherical bodies only can in all positions cast circular shadows, the earth must be round also.

(3) The 'opmasts of a vessel approaching the shore are always seen first-were the surface of the sea perfectly flat, the largest part of the vessel, the hull, would be seen first, and the small topmasts last. Navigators approaching land see the tops of the highest elevations or buildings first, and then the rest gradually come into view. The hull of a vessel leaving land disappears first; the topmasts, last. This could not be the case if the surface of the sea were flat or level.

(4) In railway cuttings, canals, &c., an allowance of eight inches per mile must be made for the dip due to the curvature of the earth's surface.

(5) By analogy. -All the other planets are round or globular in every position. The earth, itself a planet, is therefore also round.

But the earth is so large, and so little of its surface can be seen at any one time even under the most favourable conditions--as in the middle of a vast plain, or far out at sea where nothing obstructs the view-that the visible pottion of land or water, stretching away on all sides to

The Horizon, or the line where the sea or land and the sky seem to meet, appears to be flat or level. The visible portion of perfectly open land or boundless sea is certainly circular, and the higher the observer ascends, the larger the circular area within the horizon; but to the eye at the highest possible point the surface below still appears flat. If, however, the earth's surface were really flat, increase of elevation would not be followed by a corresponding increase of the visible area.4

The Earth and the Sun: Of the planets the Earth, is the third

1. More exactly, the dimensions of the earth are as follows:

Equatorial diameter. 76 miles. Polar diameter, 7899 miles. Mean diameter, zotz miles. Circua ference, 24.900 miles. Arect surface, 195,900,000! Solid content, neuly 260,000,000,0 cubic miles. Weight of the globe, about 6,000,000,000,000,000,000,000 to Is.

square miles.

2. Circumference, Lit. circun, round; Jero. I

carry.

3. Drumeter, G, dia, through; metron, a

4. At 330 feet above the sea has a radius of 9 geograpin 35 gee aaphical mines; ad qu Cal nie; at 66,000 teel, is

lev 1, the visible are t mies; at 3.300 feet, fect, 116geographi geographical indles.

in order of distance from the Sun, its mean distance being ninety-one millions of miles.

That is, there is sufficient space between the earth and the sun for 12,000 worlds such as ours side by side. Could this space be bridged over, an express train travelling night and day at a speed of a mile a minute would take over 170 years to reach the sun. It is this immense distance-400 times further off from the earth than the moon-which makes the sun appear so small, although it is in reality so large, that, supposing the sun were hollow and the earth at its centre, there would be ample space for the moon to revolve round it at a distance 160,000 miles greater than its actual distance from the earth, even though that is 240,000 miles.

Rotation of the Earth. The earth is constantly turning round, in a direction from west to east, upon an imaginary line, called its Axis, which passes through its centre, and which therefore forms one of its diameters. This motion is the cause of day and night, and is called its Daily or Diurnal Motion, because the earth rotates on its axis once a day. A very simple experiment will clearly illustrate this fact. Suspend or place a globe or ball before a lamp or candle, and observe that one-half only of the globe or ball can possibly be lit up at the same time, the other half must be in darkness. So the earth, being an opaque body and receiving its light from the one source, the sun, that half of it only that is turned towards the sun is in the light: the other half, necessarily turned away from the sun, must be in darkness. But as the earth spins or rotates unceasingly on its axis, every part of its surface is successively exposed to and turned away from the sun. Light and darkness, i.e., day and night, thus succeed each other over the greater part of the earth's surface every 24 hours.

This actual daily motion of the earth is the cause of the apparent daily motion of the sun and stars. As the earth rotates or turns on its axis from west to east, so the sun appears to pass round it in the contrary direction, i.e., from east to west, and is thus said to rise in the east and to set in the west.

The rate of rotation is easily calculated. The earth is 25,000 miles round, and as it rotates once every 24 hours, the rate of motion at the equator is a little over 1000 miles an hour, gradually decreasing to o at the poles.

This period of time any meridian on the

The exact time of rotation is 23 hours, 56 minutes, 4 seconds. is also called a sidereal day, and is the exact time required for earth's surface to come again to the same position with regard to any fixed star.

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Revolution of the Earth: The earth, while turning on its axis, also revolves round the sun once a year. This motion, called its Annual Motion, causes the Seasons.

But the earth does not move round the sun always at the same distance, because its Orbit, or path round the sun, is not a circle, but an oval or ellipse. One result of this elliptical orbit is that the earth is nearer the sun at one time than at another. Thus, on the 1st of January our globe is about 3,000,000 miles nearer the sun than on the 1st of July. The sun is therefore some 3,000,000 miles nearer the earth in winter than in summer, that is, in the northern hemisphere. This difference, however, though vast in itself, is but a fraction of the total distance, and does not sensibly affect the amount of heat and light which the earth receives from the sun. It is not to this that the great differences of heat and cold, and the varying length of day and night which accompany the changes of the seasons and the differences in latitude, are due, but to the fact that

The Earth's Axis is not perpendicular to the plane of its orbit, but is inclined to that plane at an angle of 66%1⁄2o.

1. The exact time of the earth's revolution round the sun is 365d. 6h. m. 105.

Hence the line of the Ecliptic, or apparent path of the san in the heavens, makes with the line of the earth's equator an angle of 232°.

This Inclination of the Earth's Axis never varies-in whatever part of its orbit the earth may be, its axis always points in the same direction in space.

The result is that, throughout the year, the sun is vertical to, or directly overhead, some point or other within a belt of 231⁄2 on either side of the equator, and that the poles of the earth are alternately turned towards and away from the sun. When that part of the globe in which our country is situated is inclined towards the sun, we receive much more light and heat than we do when it is turned away from the great source of both. This, then, is the cause of those differences of heat and light at different periods of the year, called the Seasons.

The Seasons: There are, in our latitude, four Seasons-Spring, Summer, Autumn, and Winter.

When the North Pole is turned towards the sun, it is Summer in the Northern Hemisphere, and when it is turned away from the sun, it is Winter.

Similarly, when the South Pole is turned towards the sun, it is Summer in the Southern Hemisphere, and when it is turned away from the sun, it is Winter.'

When the poles are neither turned towards nor away from the sun, that is, when the sun shines equally on both, and this happens twice a year, it is Spring or Autumn in the Northern Hemisphere, and Autumn or Spring in the Southern Hemisphere.*

The varying length of day and night is due to the same causes, namely, the revolution of the earth round the sun, and the inclination of the earth's axis.

But

If the earth's axis were perpendicular, or at right angles to the plane of its orbit, day and night would always be of equal length all over the world. this is not the case, for only at the equator are the day and night of equal length (each 12 hours), while at the poles the year consists of one long day of six months' duration and a continuous night of equal length. Between these extremes, the duration of day and night varies constantly, according to the season and the latitude."

Equinoxes: At two points only of the earth's orbit does the sun's light reach both poles at the same time. When this occurs, day and night are of equal duration the world over.

Hence the term Equinox (derived from the Latin æquus, equal, and nox, noctis, night). The earth reaches these two points on March 21st, which is thus the Vernal Equinox, and September 23rd, which is the Autumnal Equinox.

Solstices

Midway between the Equinoxes the earth arrives at two points called the Solstices.

1. This explains why the seasons in Australia and other countries south of the equator are exactly the reverse of those in England, and how there the heat, generally speaking, increases as we advance from south to north, and not as in Europe, from north to south.

2. Countries where the sun is vertical or nearly so have really only two seasons-a wet and a dry season. In our country, as in all temperate regions,

there are four seasons-Spring, Summer, Autumn, and Winter. In the polar regions, a brief summer is followed by a long winter.

3. At 49° 2 North or South, the longest day is 16 tours; at 63° 23′, 20 hours; at 66° 32', 24 hours. Within the polar circle, the increase in duration is rapid, from a day of one month at 67° 23′. three months at 73° 40', to six months at the poles.

At one of these points the North Polar area does not come within the Circle of Illumina tion at all, and the Northern Hemisphere generally is inclined from the sun. The earth arrives at this point, our

Winter Solstice, on December 21st. At the exactly opposite point in its orbit, the North Polar area is entirely within the Circle of Illumination, and the Northern Hemisphere generally is inclined towards the sun. This point is our

Summer Solstice, and the earth arrives at it on June 21st.

The shortest day is thus, in the Northern Hemisphere, Dec. 21st, and in the Southern Hemisphere, June 21st, while

The longest day is, in the Northern Hemisphere, June 21st, and in the Southern, December 21st.

The Tropics mark the limits on the earth within which the sun is vertical.

The earth's axis is, as we have seen, inclined to the plane of its orbit, or the ecliptic, at an angle of 66%1⁄2°. The ecliptic thus cuts the equator at two points, i.e., the sun seems to move across the equator twice a year, namely, at the Equinoxes. In other words, the sun is then perfectly vertical to, or directly overhead at, the equator. Crossing the equator on March 21st, the sun moves north, and is vertical to some point until he reaches 23° N. latitude, when he seems to stop before again returning towards the equator, which he crosses on his southward path on September 23rd, and is similarly vertical to some point south of the equator until he reaches 23° S. latitude, whence he returns again towards the equator. Hence the term Solstice, from the Latin sol, the sun, and stare,

to stand.

These points, 23° N. and 23° S. of the equator, mark the limits of the sun's apparent yearly path in the heavens, and are called the Tropics (from the Greek trepo, I turn) that north of the equator being the Tropic of Cancer, and that south of the equator, the Tropic of Capricorn. The former is thus, to the Northern Hemisphere, the summer tropic, and the latter the winter tropic, and vice versa in the Southern Hemisphere. And it is only to some point within the belt thus marked that the sun is ever perfectly vertical or directly overhead at mid-day.

Day and Night are, then, due to the rotation of the earth upon its axis. but the varying length of day and night, and consequent change of season and degree of heat and cold, are owing to the revolution of the earth, with its axis inclined to the plane of its orbit, round the sun.

Duration of Daylight: The higher the sun rises in the heavens, the longer his apparent course, hence the duration of daylight depends on the elevation or height that the sun attains in the sky. In the latitude of London, the sun rises 62° above the horizon at noon in midsummer; at noon in midwinter, it barely reaches 15-a difference of not less than 47°. On March 31st-the spring Equinox, and on September 23rd-the autumnal Equinox, the altitude or elevation of the sun above the horizon at London is 381⁄2°.

Amount of Heat: The amount of heat which the earth receives from the sun depends upon the "slant" or angle of inclination of its rays, or rather, the slant or inclination of the surface upon which the rays fall.

That is, the same number of rays, which, because of the sun's enormous distance from the earth, may be supposed to be parallel, covers more surface if

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it slants than when it is at right angles or (if we suppose the rays to be horiNow, the amount of heat received is in exact ratio to this zontal) upright. slant or angle-this is shown every day—in the early morning, or when setting, the sun's rays, falling obliquely, are much less powerful than at mid-day, when they fall more or less vertically. And the higher the sun rises in the heavens, the more vertical his rays fall, and the hotter it is. Hence, as a recent writer concisely remarks,

Vertical rays distribute the greatest quantity of heat, full torrid heat; rays at the sharpest or most acute angles, sub-tropical heat; rays at medium angles, temperate heat; and rays at the greatest angles-the least heat-giving-frigid temperature.

But the actual angle of inclination does not account for the entire difference in the amount of heat received. For the sun's rays, before they can reach the earth's surface, must pass through the atmosphere or air, which has an appreciable thickness of at least 50 miles. Air absorbs heat, and, therefore, the longer the distance that the rays have to pass through the air, the greater the amount of heat abstracted or absorbed by the air; and, similarly, the more vertical the rays fall, the less the thickness of air they pass through, and the less heat they lose. The angle of the rays and the proportionate thickness of air traversed thus explain the reason why, over and above all seasonal changes, the amount of heat received decreases generally from the equator to the poles, from the They also explain sweltering heat of tropical countries to the icy cold of polar lands. the gradual change of season, from the cool spring to the hot summer, and from the warmth of autumn to the cold of winter.

Primarily, then, all the changes that we have described, the alternation and varying duration of day and night, and the seasonal and climatic changes, are due to the daily and annual motions of the earth.

That this is so, admits of easy proof. Supposing the earth did not rotate on its axis, what would be the result? The year, or period of revolution round the sun, would consist of one long day of 6 months, and a night of equal length. But this is the actual case at the poles only. We have day and night every 24 hours. The earth, therefore, must rotate or turn round once in that time.

Again, let us suppose the earth's axis to be perpendicular to the plane of its orbit. What would happen? The ecliptic would in that case coincide with the earth's equator, day and night would be equal throughout the year all over the world, and there would be no seasonal changes-it would be perpetual winter in polar lands, perpetual spring in tem. perate regions, and perpetual summer in tropical countries. This we know is not the case; and as we experience all the changes of season in a year, the earth must revolve round the sun once a year, with its axis inclined to the plane of its orbit.

The Earth and the Moon: The earth, on its journey round the sun, is accompanied by a smaller planetary body or satellite-the

moon.

The Moon, like the earth and the other planets, is a non-luminous body, shining only by reflecting the light of the sun, and that so feebly that it would require That is, a "skysome 550,000 full moons to give us as much light as the sun. full of moons" would not give us half as much light as the sun.

The Diameter of the Moon is 2,160 miles, or bout ths of that of the earth, which is 50 times larger and 80 times heavier than its satellite.

The Motions of the Moon are very peculiar and complicated. As she revolves round the earth, she also rotates on her own axis, and, at the same time, is carried along with the earth on its journey round the sun. The earth, as we have seen, rotates on its axis once every 24 hours, and revolves round its primary, the sun, once every year. The moon, curiously enough, rotates on her own axis, and revolves round her primary, the