Thus "the planes of the orbits of the planets are subject to a variation of situation; but, after certain periods, they return to the positions from whence they departed. The inclinations of their orbits to the ecliptic are also subject to a change; but this change is confined to small limits, and in a stated period each orbit returns to that inclination from which it set off. The figure of the earth's orbit is approaching towards that of a circle; but it will afterwards gradually recede from that figure, and return to its original form. The like circumstance takes place in the figures of the orbits of the other planets. It has been found by observation, that the mean motion of the moon is increasing; but after a certain period it will decrease by the same steps, and no apprehensions need be entertained for the stability of that part of the system. The obliquity of the ecliptic is diminishing; and hence it has been supposed that seed-time and harvest, summer and winter, might hereafter cease; but it will afterwards increase, and return to its former state; and the variation will be confined to such limits, that the seasons will never be sensibly affected by it. All the planets move in the same direction in their orbits, and this is essential to the stability of the system; otherwise the disturbances would not have had their regular periods of increase and decrease, as at present, but the irregularities, by increasing, would have brought on its destruction. Hence we find no confusion of motion, which, under any other law of gravitation, would have taken place; nothing anomalous;

no constant increase of irregularity, by which, in the course of time, the system might be destroyed. There is a mean situation, about which the system oscillates through very small spaces; hence, every thing returns to that state from which it departed, and thus the whole is preserved from falling into ruin."* To this striking testimony of an eminent philosopher, whose profound knowledge of astronomy entitles his sentiments to the highest respect, I beg leave to subjoin, in conclusion, the following sentence from the pen of one not less competent to judge upon the subject: "By the most simple law, the diminution of gravity as the square of the distance increases, the planets are not only retained in their orbits, when whirling round a central sun, but an eternal stability is insured to the solar system. The little derangements which affect the motions of the heavenly bodies are apparent only to the eye of the astronomer; and even these, after reaching a certain limit, gradually diminish, till the system, regaining its balance, returns to that state of harmony and order which preceded the commencement of these secular inequalities. Even amidst the changes and inequalities of the system, the general harmony is always apparent; and those partial and temporary derangements which, to vulgar

* Vince's Confutation of Atheism, from the Laws and Constitution of the Heavenly Bodies, in Four Discourses preached before the University of Cambridge, p. 92.

minds, may seem to indicate a progressive decay, serve only to evince the permanence and stability of the whole. In the contemplation of such a scene, every unperverted mind must be struck with that astonishing wisdom which framed the various parts of the universe, and bound them together by one simple law."*

* Brewster's Edition of Ferguson's Astronomy, vol. i. p. 71, note 3, by the Editor.

QUESTIONS FOR EXAMINATION.

INTRODUCTION.

1. How is Astronomy defined?

2. What is the derivation and meaning of the term?

3. Why is it an interesting study?

4. On what ground may it be recommended, independently of its direct utility?

5. To what important art is it subservient?
6. How is it connected with Geography?
7. How has it proved useful to the historian?

8. What errors has it served to dispel?

9. What is its highest and most important use?

10. What is the observation of the Psalmist on this subject? 11. What is the sentiment of a modern poet?

CHAP. I.

1. What is the first step in the study of Astronomy? 2. What are the appearances which first present themselves to our notice in the heavens, and what inferences do we at once draw from them?

3. What do we observe on looking towards the south? 4. What do we observe on looking towards the north? 5. What remarkable point do we find there, and how is its situation marked?

6. What change takes place in the situation of the pole, when we travel northwards or southwards?

7. What should we observe if we travelled continually southwards?

8. What, then, is the appearance of the whole starry heavens, and how is it artificially represented? 9. In what respects does the celestial globe differ from the real sphere of the heavens?

10. What is represented by the wooden circle which surrounds the globe?

11. What is the derivation and meaning of the word Horizon?

12. What is meant by the terms Zenith and Nadir? 13. How may we represent on the celestial globe the aspect and motion of the heavens, as seen at any place? 14. What are the use and probable origin of the various figures which are drawn upon the surface of the globe ?

15. Mention and point out on the globe some of the more remarkable constellations.

16. How are the stars further distinguished and classified? 17. What method is employed for designating particular stars, and by whom was it invented?

CHAP. II.

1. What are the chief points of the horizon, and how are we led to remark them?

2. What is the first great circle which the ancient astronomers imagined to be drawn upon the sphere, and how is it situated ?

3. What is the next great circle, and how is it situated? 4. Why is a star said to culminate, when it crosses the

meridian?