11. Celestial globes have sometimes been made so large that the student could go within, and see from the centre the constellations; which, in this case, were painted on the inner or concave surface. This would give him a correct idea of their positions. If we suppose that such a large globe stood under the open sky, that the painting was transparent, and the globe rectified for the place and the hour, then the observer at the centre might look through the painted constellations of his transparent globe directly to the corresponding real constellations in the heavens. If, for example, he looked towards the north, and through the Great Bear drawn on his globe, he would see in the heavens, not the figure of a bear, it is true, but the six beautiful stars, which, both on the globe and in the heavens, form the well-known Dipper; and he would see the two stars in the front, called on the globe Merac and Dubhe, pointing to a smaller star called Cynosura in the Little Bear; around which, if his great concave globe were slowly turned from east to west, to imitate apparent diurnal motion, he would see the Dipper and the other stars revolve; and this is the POLAR-STAR.

12. The cultivation of astronomy goes back to the earliest dawn of

history. When Alexander the Great took Babylon, B.C. 331, he found reason to believe that this first of human sciences had begun in Chaldea, antecedent to the founding of the Hebrew nation by Abraham. In the Book of Job, one of the very earliest writings of antiquity, the most beautiful constellation of the heavens, Orion (see fig. 4), is called by its present name, as is also Arcturus, a star of the first magnitude, now ranged in the constellation Bootes, and the most interesting group of the celestial sphere, "the Pleiades," in Taurus.

The constellation Orion contains seventy-eight stars. The general appearance will be seen by reference to the engraving in p. 9. The outline of a figure is said to be formed thus:-The three upper stars form the head; two larger ones, the shoulders and arms; three similar stars, a belt or girdle; smaller stars form the lower limbs; and two large ones at the bottom, the two feet.

13. Why the beautiful stars were arranged according to such fantastic shapes as we see on the celestial globe, cannot in all cases be decided. But there is a significancy in many of the names and figures; and others commemorate heroes and heroines familiar to ancient story. Neither can the origin of the names of countries always be traced, yet the geographer avails himself of them as they exist. So is it with the astronomer in regard to the constellations. Their odd names and strange shapes may serve to fix them in his memory.

QUESTIONS UNDER CHAPTER II

1. Which of the two globes is most easily understood, and why?

2. In what respects are the two globes different? What is here said of the concave of the heavens ?

3. What do the uncouth figures on the celestial globe represent? What are constellations?

4. What is said of fixed stars? Of wandering stars? Since they are not placed on celestial maps, how can their course be made intelligible?

5. Is the sun's motion real, or only apparent? What is said of the ecliptic? What is the zodiac?

6. Where, in regard to the earth, are the fixed stars? How do the fixed stars differ from the planets in regard to their light? What is said of the fixed stars having, like our sun, opaque planets revolving around them?

7. What is said of the distances of the fixed stars? What is said concerning a binary star in the Swan? What fact is established? What is said of Sirius?

8. How are the fixed stars divided? How are the visible stars ordinarily divided? How many are there of the first magnitude, and what is said of them?

9. Of the visible fixed stars how many have been counted? stars? How many?

What are telescopic

10. What are nebule? What have these been found to be, and by whom? What is said of the Milky Way?

11. What if a transparent celestial globe were made so large that an observer could go into the earth's place at the centre? What would such an observer see at the north? 12. What is said of the date of the science of astronomy? What evidence have we that it begun as early as the date of the Book of Job?

13. What is said of the origin of the constellations? Of their use ?

EXERCISES.

Observe on the globes whatever they are designed to represent, especially when connected with the present lesson.

The hour circle on the artificial globe is a small circle, sometimes of brass, with an

index or pointer fixed to the north pole; it is divided into twenty-four equal parts, to represent the twenty-four hours of the day. The noon, XII., is on the upper or superior part of the meridian, and the midnight, XII., is on the lower or inferior part of the meridian.

The wooden horizon represents the true or rational horizon, and has marked upon its surface several circles. One contains the twelve signs of the zodiac, through which the sun appears to revolve in a year. Another circle contains the signs of the zodiac, divided into degrees and the days of the month to which they correspond.

A quadrant of altitude is a thin slip of brass divided into 90 degrees, and may be screwed on to the brass meridian at the zenith. It is used to measure distances.

Set the north pole of the celestial globe towards the north. Elevate it above the wooden horizon a number of degrees equal to your latitude. The north pole star will then be in the situation in which you will see the polar star in the heavens every cloudless night; and the equinoctial on the globe will have the same slope at which, in your latitude, it always appears. There are stars which mark the imaginary, though determinate place of this circle. They will each, in its turn, appear and disappear; but the line is ever in the same place, and will ever make with your horizon an angle which is the complement of your latitude. Let the student fix the place of the equinoctial well in his mind, by learning as fast as possible, from the celestial globe and the actual heavens, the constellations through which it passes, and the single bright stars, or the groups of stars, on or near it. Observe the sun's course on the 20th of March and September-at the time of the equinoxes. The line he then describes is the equinoctial.

Find on the celestial globe the following constellations named in the preceding chapter: Ursa Major, the Great Bear; Ursa Minor, the Little Bear; Cygnus, the Swan; Taurus, the Bull; Canis Major, the Great Dog; Orion; and Boutes, the Herdsman, with his Dogs. Which of them does the equator pass through, and which the ecliptic? What stars of the first magnitude do you find on or near the equator? What on or near the ecliptic? Name and describe them.

Where is Cynosura? Where is Sirius? And how many degrees is it from the equator? (Measure by passing it under the brazen meridian.)

Learn from the terrestrial globe the position of the tropic of Cancer. seas and countries of America does it pass? Through what of Africa? are Canton and Calcutta situated with respect to the tropic of Cancer? situated with respect to the tropic of Cancer?

Through what Of Asia? How How is Havana

CHAPTER III.

THE STUDIES OF THE CELESTIAL GLOBE MAY BE PURSUED BY OCULAR EXAMINATION OF THE HEAVENS-THE VIEW ON THE EARTH LIMITED THE SENSIBLE HORIZON-THE SOLAR SYSTEM-THE PRINCIPAL PLANETS AND ASTEROIDS THE SATELLITESCOMETS THE SUN, AND ITS INFLUENCES-SIR JOHN HERSCHEL'S ILLUSTRATION OF THE COMPARATIVE SIZES OF THE BODIES OF THE SOLAR SYSTEM.

1. IN the studies of the earth and heavens, by means of the terrestrial and celestial globes, there is this further difference. The student having, by the aid of his celestial globe, with such assistance from the living teacher as he may be able to obtain, learned to know the principal constellations, with their most remarkable stars, can then go forth at evening and pursue his studies, by tracing out his subjects upon the starry concave of the blue vault above. There, by the remarkable stars through which they pass, he can locate the circles of the ecliptic, the equinoctial or equator of the heavens, the two colures, and the other remarkable positions of astronomy. But we cannot, in the same manner, trace out upon the earth's real sur

face the places where are the equator, the tropics, or any of the imaginary circles which belong to geography.

2. For the SENSIBLE HORIZON, which is the boundary of the actual view, either on land or water, which the human eye can at once take in, is extremely limited. In water, or on a level plain, it extends only three miles each way from the observer, whose eye is raised five feet from the surface. Its extent increases in proportion to the observer's elevation. Let him be raised 100 feet aboye the surface, and his view will extend ten miles in every direction. From Dwalghiri, the highest peak of the Himalaya mountains, 28,000 feet high, if the view were unobstructed, the observer's eye might faintly see at the distance of 147 miles. This we infer from the fact that this peak is seen at that distance.

3. Sir John Herschel argues from these facts that our sight may convince us of the convexity of the earth, wherever, at sea or on land, the view is unobstructed. Every particle of solid matter, and every drop of water, alike gravitate to the centre of the earth; thus forming around it a globe. The particles of water, moving among themselves, settle after being agitated, and the unruffled ocean is like one vast convex mirror. An observer's eye being placed close to its surface, or to that of a vast level on land, his vision would extend in a plane tangent to (or touching) the earth at the place of his eye. This plane is parallel to the rational horizon, and is always meant when the plane of the sensible horizon is mentioned. But let the observer's eye be elevated, and the line of his vision will no longer lie in this tangent plane, but his sensible horizon will droop or dip. Imagine a line drawn from the elevated eye to the convex surface of the earth, and passing onwards to the concave heavens, and then this line revolved,-where it touched the earth it would describe the extreme circle of the observer's terrestrial view, and where it met the heavens, describe the extreme circle of the observer's celestial view; and both these circles will be enlarged in proportion to the observer's elevation, and the consequent increased dip of the horizon. The line revolved produces an imaginary cone.

Fig 5.

4. The earth, on account of its convexity, gradually recedes beyond the circle of terrestrial vision. We will suppose a ship at sea, yet below the circle, but approaching the observer, the top of the mast will first rise above his dipping horizon, and be seen; then the lower parts of the masts; and last of all there heaves in sight the hull, or body of the ship, which is its largest part (see fig. 5). Since these phenomena occur in

every part of the world, and since nothing but a convex figure could produce them, the earth must therefore be everywhere convex; that is, it must be a globe. To such certainty and practical exactness are these principles reduced, that the height of an object is calculated from its distance, or its distance from its height.

5. It is evident that heavenly bodies will appear in different positions, viewed by observers on different parts of the earth's surface; and hence that observations made at any one place on the surface will not be of general use. Astronomers, to rectify this inconvenience, have adopted the method of making all calculations upon heavenly bodies as if all saw them from the same spot; and this one imaginary point of astronomical view is the centre of the earth, 4,000 miles from its surface.

6. And where is the EARTH? It is floating in space; but not like a leaf in the wind. For not more has the railroad an appointed track than has the earth its path in space. To other worlds-and such we deem the stars to be the earth appears as a star. It belongs to a group of celestial bodies, of which the SUN (or Sol, as called by the Latins) is the great central source of light, of heat, and of attraction.

7. Of the bodies which belong to the solar system there is, in the first place, the great SUN itself, whose diameter is 887,000 miles. In the second place, there are the PRIMARY PLANETS, of which eight are principal planets, whose names, in the order of their distance from the sun, are as follows: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus or Herschel, Neptune or Leverrier.

8. Besides these, there are, between Mars and Jupiter, several small and newly discovered planets, called Asteroids. These are also called primary planets, as they revolve directly about the sun; and are thus distinguished from the next or third class of bodies belonging to the solar system, which are termed SECONDARY PLANETS, moons, or satellites. These revolve around certain of the primary planets, and are at the same time carried with them around the sun. Of the secondary planets there are at least twenty-one. Of these

9. The COMETS are the fourth class of bodies belonging to the solar system. They revolve about the sun in orbits very elliptical. They are divided into interior and exterior. The interior comets are those whose orbits are at their aphelion, or greatest distance from the sun, wholly within the range of that of Neptune, the outermost of the known planets. Of these there are six whose orbits and periodic times have been calculated, and which return in periods of from three to seven years. Of the exterior comets, the number is unknown, but