Let us take another view of our diagram, page 30. Here we have represented the position of the earth with regard to the sun on the 21st of June, the 22nd of September, the 21st of December, and the 20th of March—that is, during the summer and winter solstices, and the vernal and autumnal equinoxes; and in each position of the earth it is evident that its axis points in the same direction, moving, as is said, parallel to itself.

At A, or during the summer solstice, the sun is perpendicular to the TROPIC OF CANCER, or to that part of the earth's surface which is 23 degrees north of the equator; and as he always shines over ninety degrees in every direction, from the point over which he is perpendicular, it is evident that his light will be diffused 23 degrees over and beyond the north pole, while the same extent of surface round the south pole will be deprived of his rays. And hence, as in the diagram, the entire Arctic circle comes within, while the Antarctic lies without the circle of illumination; and it is for this reason that these circles are described 23 degrees distant from each pole.

The earth proceeds in her course, and in three montlis after is in the position represented at B. Here the sun, which, in consequence of the earth's motion in her orbit, has been daily withdrawing his rays from about the north pole, is perpendicular to the equator-and the circle of illumination, consequently, extends from pole to pole.

At C in the diagram, the winter solstice is represented. In this case the rays of the sun are perpendicular to the TROPIC OF CAPRICORN, and the circle of illumination consequently sweeps round the south pole, and 23 degrees beyond it, leaving the north pole and 23 degrees around it (that is, the entire north frigid zone) involved in darkness.

At D the earth has arrived at the vernal equinox, and the sun is again perpendicular to the equator, and the circle of illumination again extends from pole to pole.

But how do we know that the earth moves round the

along them, they will all seem to point directly to the moon in the horizon, which, of course, is occasioned by the great distance of the moon from us: and, perhaps, the distance between the lines (10 or 15 feet) will bear as great a propcrtion to the distance of the moon from the earth (240,000 miles), as 190.000,000 of miles to the distance of the polar star.

sun? Besides the sun's apparent diurnal motion from east to west, he appears, when closely observed, to move nearly a degree, or about twice his own diameter, every day to the eastward, and thus, in the course of a year, to complete a great circle in the heavens. But his apparent diurnal motion has led us to doubt the evidence of our senses in these matters, and a little reflection will convince us that his annual motion also is merely apparent, and that the circle which he seems to describe in the heavens is really described by the earth in travelling round her orbit. In illustration of this, place a globe or candle on a table in the middle of a room,. and move round it in a circle, keeping your eye upon it and the wall in a line with it. In this case the globe or candle, though fixed in the middle of the room, will appear to you to describe a circle round the wall. And thus the sun,. though at rest in the centre, appears to us to describe a circle round the heavens, which is really described by the earth in its orbit; the earth describing one part of the circle while the sun appears to describe the opposite.

The circle which the sun thus appears to describe in the heavens among the fixed stars, is called the ECLIPTIC; be

"If, by means of a telescope, or through the shaft of a very deep mine, the sun be observed in a line with a fixed star, the next day, at the same hour, he will appear to have moved nearly a degree, or about twice his own diameter to the east of such star; and in twenty-four hours more, another degree eastward, and so on, till having completed a great circle in the heavens, he returns in the course of a year, to the same fixed star. Observations to the same effect may be made on any clear evening after sunset. If a star be observed near the horizon to the eastward of the place where the sun appeared to set, the next evening, at the same time, it will appear nearer to the place where the sun disappeared, and on the next still nearer, and so on till it sets alongwith the sun, and is consequently lost in his rays.

Thus, when the earth is in Libra, the sun appears to be in the op-posite sign, Aries; and when the earth moves to Scorpio the sun seems to enter Taurus, and so on.

Ecliptic.-As this circle was supposed to be described by the sun in 360 days, the ancent astronomers divided it into three hundred and sixty equal parts, which they called DEGREES to denote the (steps); progress made each day by the sun; and hence the division of the cir cumference of ALL CIRCLES, great and small, into three hundred and sixty equal parts or DEGREES. The ecliptic is also divided into twelve equal parts, containing thirty degrees each, to correspond to the twelve months of the year. These parts are called the SIGNS of the ZODIAC, because they are generally represented by the signs or figures of animals. The term ZODIAC, which is derived from a Greek word signifying animal, is applied to a broad circle or belt in the heavens, extending about eight

cause as it is in the same plane with the earth's orbit, an eclipse will take place when the moon comes within it.

As an illustration of this, let the circle which your head, in moving round the globe just referred to, may be supposed to have described, represent the orbit of the earth, and let the circle apparently described by the globe round the walls of the room represent the ecliptic, and it will be evident that these circles have the same centre and lie in the same plane. Now hold up an orange or a ball in a line between your eye and the centre of the globe and you will have a representation of an eclipse. The globe is supposed to represent the sun, your head the earth, and the orange the moon; and, as they all lie in the same plane, it is evident that the orange will intercept a portion of the globe from your view. This is a representation of an eclipse of the sun; and if you turn your back to the globe, keeping the orange in the same line with it and your head, you will have a representation of an eclipse of the moon-for in these cases, your head will intercept the light supposed to come from the globe to the orange. If you raise or lower your hand no eclipse will take place; because, in these cases the orange would be either above or below the plane of the ecliptic. And it is in this way that the moon generally moves, either a little above or a little below the plane of the earth's orbit, otherwise there would be two eclipses every month-one of the sun at new moon, and another of the moon at full moon.

degrees on each side of the ecliptic. In this circle the earth and most of the planets revoive. The newly discovered planets, however, and some of the Asteroids, are more than eight degree snorth or south of the ecliptic, and thus without the Zodiac. The names of the twelve signs, with their symbols, are—

The sun rises in Aries on the 21st of March, and in a month after in Taurus, and so on through the signs in monthly succession.

The signs of the Zodiac are trigonometrical divisions of the Ecliptic commencing at the actual Equinox, and must not be confounded with the constellations, called Aries, &c.

a Eclipse. This term is derived from a Greek word, which signifies a leaving out or deficiency; of course, in this case, of light. Ellipse is from the same root, namely EK, out, and LEIPO (eipso), to leave.

QUESTIONS FOR EXAMINATION ON CHAPTER II.

Pages 23-26.-How illustrate the diurnal motion of the earth? 2. Why is one-half of the earth always enlightened? 3. Why day and night alternately? 4. Why in the course of twenty-four hours? 5. What is meant by the term poles? 6. Why the north pole called upper? 7. How illustrate the poles by making an orange or top spin round? 8. Why do the sun and heavenly bodies appear to revolve from east to west in twenty-four hours? 9. How illustrate these apparent motions? 10. What are the arguments against the supposition

that the sun revolves round the earth? 11. If the sun revolves round the earth every twenty-four hours, what must be the circumference of the circle he would have to describe daily? 12. How do you show this? 13. What idea can you give of the number of the fixed stars? 14. What, of their distance? 15. What the inference from these facts? 16. How much is the sun larger than the earth? 17. Is there any thing known of the magnitude of the fixed stars? 18. What are they supposed to be? 19. A planet appears brighter and larger if viewed through a telescope; is this the case with a fixed star? 20. How do you account for this?

6.

Pages 27-30.-The seasons caused by? 2. What is meant by the orbit of the earth? 3. The plane of the earth's orbit? 4. How illustrate them? 5. Are we to suppose that the orbit of the earth is a solid or substantial ring, or that its plane is a real, visible, flat surface? How illustrate what has been said? 7. The equator? 8. Why so called? 9. Into what does it divide the globe? 10. In what direction does it run? 11. If the axes of the earth were not inclined to the plane of its orbit, what would be the consequence? 12. What is the measure of the angle made by the axis of the earth with the plane of its orbit? 13. What do you understand by the axis moving parallel to itself? 14. What is the consequence of this? 15. When the nor thern half of the axis is inclined to the sun, what is the consequence? 16. When the axis of the earth neither inclines to nor declines from the sun, what is the consequence? 17. How illustrate what has been said? Pages 30-32.-Can you explain the diagram in this page? 2. In what position is the earth with regard to the sun at midsummer? 3. Midwinter and the equinoxes? 4. When the sun is vertical to the tropic of Cancer, how far does he shine over the north pole? 5. Why? 6. Över what part of the earth is the sun vertical, when his rays are withdrawn from the north frigid zone? 7. When the sun is vertical ten degrees north of the equator, how far does he shine over and beyond the north pole? 8. In this position of the earth, how much of its surface round the south pole is deprived of his light? 9. Over what part of the earth is the sun vertical, when he shines from pole to pole? 10. What is a great circle ? 11. The illustration given in the note ? 12. What is meant by the circle of illumination? 13. Why does it always bisect the equator? 14. The consequence of this? 15. At what periods of the year are the parallels bisected by the circle of illumination? 16. Why at those periods are the days and nights equal all over the world? 17. When the sun is north of the equator, is there more or less than half of each of the parallels within the circle of illumination? 18. When the days are sixteen hours long at any place, how much of the parallel of latitude of that place is within the circle

20.

of illumination? 19. How illustrate the circle of illumination? In what position would the earth be if the circle of illumination coincided with the equator?

Pages 33-36. The advantage of getting a clear idea of what is meant by the circle of illumination? 2. Can you explain how it is possible that the pole of the earth always points in the direction of the pole of the heavens, while travelling round in an orbit one hundred and ninety millions of miles in diameter? 3. Over what part of the earth is the sun vertical on the 21st of June? 4. On the 20th of March? 5. On the 22nd of September? 6. During the winter solstice? 7. Why are the Arctic and Antarctic circles described at the distance of 23 degrees from the poles? 8. How may the sun's apparent motion to the eastward be observed? 9. What progress does he appear to make every twenty-four hours? 10. When the earth is in Libra, in what sign is the sun? 11. When the earth is in Scorpio, in what sign is the sun? 12. The ecliptic? 13. Why so called? 14. How illustrate it? 15. What is an eclipse? 16. How illustrate the principle of an eclipse of the sun? 17. Of the moon? 18. The origin of the division of the circle into the 360 degrees? 19. What is the zodiac? 20. The signs of the zodiac? 21. Can you repeat the twelve signs? 22. When is the sun in Aries? 23. When in Taurus? 24. When is the sun in Libra ? 25. In what sign is the earth when the sun is in Capricorn? 26. Why are there not two eclipses every month?

CHAPTER III.

CAUSES OF THE EARTH'S ANNUAL MOTION.

We shall now briefly explain the causes of the earth's motion round the sun. Let us suppose that the earth at its creation was projected forward into pure and boundless space, and it is certain that if no obstacle occur to impede its course, it will move on in a straight line and with a uniform

It requires no argument to prove that, if a body is at rest, it must always remain so, unless moved from its position by the application of some force or power; but it appears strange and incredible, that if a body is set in motion, it must move on for ever, unless some obstacle occur to prevent it. This is clearly proved in the following illustration of an argument from Archbishop Whately's Rhetoric, page 70:

"One part of the law of nature, called the 'vis inertiæ,' is established by the argument alluded to (The Argument of Progressive Approach): viz., that a body set in motion will eternally continue in motion with uniform velocity in a right line, so far as it is not acted upon by any causes which retard or stop, accelerate or divert its course. Now, as in every case which can come under our observation, some such causes do intervene, the assumed supposition is practically impossible, and we have no opportunity of verifying the law by direct experiment; but we may gradually approach indefinitely near to the case supposed; and on the result of such experiments our conclusion is founded. We find that when a body is projected along a rough surface, its motion is