ESSAY. INTRODUCTORY REMARKS.

In the whole range of Astronomy, there is no subject so productive of curious speculation, or presenting so wide a field for discovery, as that department of the science which relates to Comets. These bodies, from the striking peculiarities of their movements, no less than the anomalous nature of their physical constitution, of late years have engaged the especial attention of philosophers; and the interest thus excited on their own account, is yet farther increased, by the important information which they communicate concerning the extent and constitution of the System. But though it is only recently that Comets have begun to direct the labours of the astronomer, they have for a long period attracted the general observation of mankind—not so much, however, as the objects of science, as objects of superstitious terror. Suddenly presenting themselves in the firmament, and dragging after them a luminous train which often stretches to a prodigious distance, but continuing visible only for a short time and then vanishing from mortal sight, Comets, in earlier ages of the world, were conceived to possess a malignant influence over human affairs—portending the occurrence of wars and of famines—foreboding the dethronement of monarchs, and the dissolution of empires—causing, in short, every calamity and disaster “which flesh is heir to.” But, like most other prejudices, which gradually disappear on the advancement of knowledge, the absurdity and extravagance of these opinions have at length been fully exposed. The discoveries of Astronomy have shown, that Comets form only component parts of the solar system, being constituted of like materials, and guided in their course by the same laws of attraction, as the Planets themselves; so that unless this moral influence belongs likewise to the Planets, it cannot justly be ascribed to Comets, which hold so many essential characters in common with the former.

But notwithstanding the general identity of character which exists between the Planets and Comets, as members of the same system, it will be found that in other respects they form distinct classes of bodies, and that the Comets are not only vastly more numerous, but occupy a far more important place in the economy of the system. The planets in their revolutions are confined to a narrow zone in the heavens, but comets range freely through every part:—the planets as well as their satellites, move only in one particular direction; but comets are not limited to one direction more than to another:—the planets are constrained to revolve in orbits which allow them to vary little in their distance from the sun; but comets follow paths extremely eccentric, traversing the planetary orbits, and even wandering beyond the known limits of the system. The number of planetary bodies, including the satellites, is no more than twenty-nine; whilst the number of comets actually observed exceeds four hundred, and the whole number which permeate the system must amount to many thousands. If, then, in addition to those circumstances, we consider their singular appearance, differing so widely from that of all the other heavenly bodies, it must be admitted that Comets form a subject of philosophical investigation, highly interesting, as much on account of their own striking peculiarities, as of the general views respecting the planetary system which they will be found to suggest.

Yet curious and important as it is, there is no branch of Astronomy in which slower progress has been made, or in which less of certainty prevails. The subject, indeed, is one in itself of so much real difficulty, and the prejudices which stood in the way of discovery have been but so recently removed, that this circumstance need not perhaps excite our surprise. Much time and observation are required before a sufficient number of data can be obtained; and without those data as a groundwork, any theories which we attempt to form respecting Comets, can be only empirical. Within these few years, however, the assiduity of astronomers in almost every quarter of the globe, has added a multitude of facts to the store formerly obtained;—facts which, while they demonstrate the fallacy of many of the opinions generally held respecting comets, now enable us to make very considerable approaches towards a knowledge of the true nature of these singular bodies.

In a science like that of astronomy, whose advancement depends so much on the result of experience, it is necessary that we should frequently review the progress already effected, class according to certain principles the new facts which may have been acquired, and observe the general inferences to which their discovery leads. If this remark be true as to Astronomy in general, it applies with peculiar force to that department of the science upon which we are now entered; and this is the object which in the following Essay I shall endeavour to keep mainly in view.

It may be proper here to premise very shortly the arrangement which has been adopted. The extraordinary appearance of Comets being what first and most forcibly excites our attention, I shall begin with describing the physical constitution of Comets, as far as the latest discoveries have ascertained. The next consideration naturally is the course which these bodies follow in moving through the heavens; and in this part of the subject, the nature of their orbits, together with the methods of determining the elements of the orbits, will be explained. The perturbations to which all the bodies in the system are more or less exposed by foreign attraction, apply in an especial manner to comets; and therefore, the consideration of the mutual influences of the Comets and Planets on one another, will constitute the third division of the Essay. A fourth part will be devoted to several speculative inquiries relative to the different stages of maturity which Comets may be supposed to have attained. It has been already remarked, that Comets, both from their prodigious number, and their extensive movements through the system, are calculated to suggest many important views with respect to the planetary system in general. The consideration of these views will form the fifth and concluding part of the Essay.