INTRODUCTION.

SCIENCE, regarded as the pursuit of truth, must ever afford occupation of consummate interest, and subject of elevated meditation. The contemplation of the works of creation elevates the mind to the admiration of whatever is great and noble; accomplishing the object of all study, which, in the elegant language of Sir James Mackintosh," is to inspire the love of truth, of wisdom, of beauty,-especially of goodness, the highest beauty, and of that supreme and eternal Mind, which contains all truth and wisdom, all beauty and goodness. By the love or delightful contemplation and pursuit of these transcendent aims, for their own sake only, the mind of man is raised from low and perishable objects, and prepared for those high destinies which are appointed for all those who are capable of them."

In tracing the connection of the physical sciences, astronomy affords the most extensive example of their union. In it are combined the sciences of number and quantity, of rest and motion. In it we perceive the operation of a force which is mixed up with every thing that exists in the heavens or on earth; which pervades every atom, rules the motions of animate and inanimate beings, and is as sensible in the descent of a rain drop as in the

B

falls of Niagara, in the weight of the air as in the periods of the moon. Gravitation not only binds satellites to their planet, and planets to the sun, but it connects sun with sun throughout the wide extent of creation, and is the cause of the disturbances, as well as of the order, of nature: since every tremour it excites in any one planet is immediately transmitted to the farthest limits of the system, in oscillations, which correspond in their periods with the cause producing them, like sympathetic notes in music, or vibrations from the deep tones of an organ.

The heavens afford the most sublime subject of study which can be derived from science. The magnitude and splendour of the objects, the inconceivable rapidity with which they move, and the enormous distances between them, impress the mind with some notion of the energy that maintains them in their motions, with a durability to which we can see no limit. Equally conspicuous is the goodness of the great First Cause, in having endowed man with faculties, by which he can not only appreciate the magnificence of his works, but trace, with precision, the operation of his laws, use the globe he inhabits as a base wherewith to measure the magnitude and distance of the sun and planets, and make the diameter of the earth's orbit the first step of a scale by which he may ascend to the starry firmament. pursuits, while they ennoble the mind, at the same time inculcate humility, by showing that there is a barrier which no energy, mental or physical, can ever enable us to pass: that, however profoundly we may penetrate the depths of space, there still remain innumerable systems, compared with which, those apparently so vast must dwindle into insignificance, or even become

I Note 1.

Such

invisible; and that not only man, but the globe he inhabits,-nay, the whole system of which it forms so small a part,―might be annihilated, and its extinction be unperceived in the immensity of creation.

It must be acknowledged, that a complete acquaintance with physical astronomy can be attained by those only, who are well versed in the higher branches of mathematical and mechanical science1, and that they alone can appreciate the extreme beauty of the results, and of the means by which these results are obtained. It is nevertheless true, that a sufficient skill in analysis 2 to follow the general outline,-to see the mutual dependence of the different parts of the system, and to comprehend by what means some of the most extraordinary conclusions have been arrived at,—is within the reach of many who shrink from the task, appalled by difficulties, which, perhaps, are not more formidable than those incident to the study of the elements of every branch of knowledge. There is a wide distinction between the degree of mathematical acquirement necessary for making discoveries, and that which is requisite for understanding what others have done.

All the knowledge we possess of external objects is founded upon experience, which furnishes facts; and the comparison of these facts establishes relations, from which, induction, that is to say, the belief that like causes will produce like effects, leads to general laws. Thus, experience teaches that bodies fall at the surface of the earth with an accelerated velocity, and with a force proportional to their masses. By comparison, Newton proved that the force which occasions the fall of bodies at the earth's surface, is identical with that which retains the

moon in her orbit; and induction led him to conclude, that as the moon is kept in her orbit by the attraction of the earth, so the planets might be retained in their orbits by the attraction of the sun. By such steps he was led to the discovery of one of those powers, with which the Creator has ordained, that matter should reciprocally act upon matter.

Physical astronomy is the science which compares and identifies the laws of motion observed on earth, with the motions that take place in the heavens; and which traces, by an uninterrupted chain of deduction from the great principle that governs the universe, the revolutions and rotations of the planets, and the oscillations1 of the fluids at their surfaces; and which estimates the changes the system has hitherto undergone, or may hereafter experience, changes which require millions of years for their accomplishment.

The accumulated efforts of astronomers, from the earliest dawn of civilisation, have been necessary to establish the mechanical theory of astronomy. The courses of the planets have been observed for ages, with a degree of perseverance that is astonishing, if we consider the imperfection and even the want of instruments. The real motions of the earth have been separated from the apparent motions of the planets; the laws of the planetary revolutions have been discovered; and the discovery of these laws has led to the knowledge of the gravitation2 of matter. On the other hand, descending from the principle of gravitation, every motion in the solar system has been so completely explained, that the laws of any astronomical phenomena that may hereafter occur, are already determined.

SECTION I.

ATTRACTION OF A SPHERE. FORM OF CELESTIAL BODIES. TERRESTRIAL GRAVITATION RETAINS THE MOON IN HER ORBIT. -HEAVENLY BODIES MOVE IN CONIC SECTIONS. GRAVITATION PROPORTIONAL TO MASS.GRAVITATION OF THE PARTICLES OF MATTER. FIGURE OF THE PLANETS. HOW IT AFFECTS THE MOTIONS OF THEIR SATELLITES.-ROTATION AND TRANSLATION IMPRESSED BY THE SAME IMPULSE. MOTION OF SUN AND SOLAR SYSTEM.

IT has been proved by Newton, that a particle of matter1, placed without the surface of a hollow sphere2, is attracted by it in the same manner as if the mass of the hollow sphere, or the whole matter it contains, were collected into one dense particle in its centre. The same is therefore true of a solid sphere, which may be supposed to consist, of an infinite number of concentric hollow spheres.3 This, however, is not the case with a spheroid4; but the celestial bodies are so nearly spherical, and at such remote distances from one another, that they attract and are attracted as if each were condensed into a single particle situate in its centre of gravity5,—a circumstance which greatly facilitates the investigation of their motions.

If

Newton has shown that the force which retains the moon in her orbit, is the same with that, which causes heavy substances to fall at the surface of the earth. the earth were a sphere, and at rest, a body would be equally attracted, that is, it would have the same weight

1 Note 6.
4 Note 9.

2 Note 7.
5 Note 10.

3 Note 8.