mena accompanying their transits and occultations are also suggestive of some interesting speculations.

This chapter introduces a name which occupies a prominent place in the remaining portion of the work-it is the immortal name of William Herschel. To the bulk of the intelligent class of readers this illustrious individual appears in the character of an astronomer distinguished by his skilful construction of huge telescopes, which he employed with marvellous success in exploring the heavens. To the student who has advanced within the precincts of astronomical science, he forms a more exalted object of admiration, as an observer of almost unrivalled acuteness and sagacity, whose exquisite faculty of discernment frequently enabled him to arrive at results far beyond the scope of the mere instrumental resources available to him; and as a philosopher of the highest order, who, by his originality of thought and capacity for comprehensive speculation, succeeded in establishing the principles of Sidereal Astronomy upon a broad and indestructible basis.

The fifteenth chapter contains an account of the progress of enquiry on the physical constitution of Comets. These mysterious bodies, beyond doubt, perform some important function in the economy of nature, which can only be ascertained by attentive observations of the phenomena which accompany their various apparitions.

The sixteenth chapter is devoted to those physical principles whose influence in disturbing the apparent positions of the celestial bodies, or in modifying the features of celestial phenomena, must necessarily be taken into account before astronomical observations can be rendered available as the groundwork of ulterior enquiry. It comprehends an account of the progress of researches on Precession, Refraction, Aberration, Nutation, Diffraction, and Irradiation. In the history of Refraction the mighty names of Newton and Laplace reappear with transcendant lustre. The correspondence between Newton and Flamsteed, published by the late Mr. Baily, has supplied some interesting materials connected with the researches of Newton on this subject. The uncertainty which so long existed respecting the construction of Newton's table of refractions, which Halley originally communicated to the Royal Society-whether it was based upon some physical theory of the subject, or whether it was calculated merely by an empiric process-has been effectually removed by the correspondence above referred to. It appears that Newton studied profoundly the theory of Astronomical Refraction, and succeeded in determining the results corresponding to various hypotheses respecting the physical constitution of the atmosphere. His suggestion to Flamsteed, recommending the practice of noting the indications of the barometer and thermometer, as a desirable accompaniment to astronomical observations, constitutes a striking illustration of the sagacity by which that great philosopher was distinguished above ordinary enquirers.

The subjects of Aberration and Nutation are introduced, with an account of the original discovery of these phenomena by the immortal

Bradley. The chapter closes with an account of the most important results which have been elicited by the labours of successive enquirers on the subject of the Irradiation of Light.

The seventeenth chapter is devoted to the history of the physical enquiries connected with eclipses of the Sun and Moon, the transits of the inferior planets, and other occurrences of a similar character. These phenomena are all influenced in so great a degree by Refraction, and the other affections of light, that it would have been inconvenient to have alluded to them at an earlier stage of the work. This chapter contains a somewhat detailed exposition of the most important facts which have been observed on the occasion of the various total eclipses of the Sun recorded in history, including an investigation of the conclusions which they are calculated to suggest respecting the physical constitution of the great central body of the planetary system. The subject of the transits of Venus has naturally suggested a brief notice of the life and labours of the lamented Horrocks. There is a deep interest associated with the fate of this youthful astronomer. Although dwelling in a remote district of Lancashire, in almost entire seclusion from the rest of the scientific world, he unquestionably arrived at a juster appreciation of Kepler's discoveries than any of the successors of that great astronomer had hitherto done; while the sagacity and originality of his views on various points relating to astronomy, his fertile and glowing imagination, and his ardent enthusiasm in the pursuit of science-all seemed to foreshadow a career of uncommon brilliancy, which a premature death unfortunately soon brought to a close. Even his brief labours, however, have assured to him a reputation which will live imperishably in the annals of science. By his own countrymen he cannot fail to be regarded with peculiar interest, as the morning star of a galaxy of men of genius, who continued for about a century to adorn these isles by their successful cultivation of the physico-mathematical sciences. The names of Horrocks, Gascoigne, Brouncker, Barrow, Wallis, Wren, Gregory, Hooke, NEWTON, Taylor, Bradley, Simpson, and Maclaurin, represent a constellation of scientific enquirers, which for splendour of genius and high intellectual endowments has never been surpassed, and rarely equalled, during a similar period in the history of any nation, whether of ancient or modern times.

As any work relating to astronomical science would be incomplete without some allusion to the important subject of observation, the eighteenth chapter has been devoted to a condensed account of the progress of Practical Astronomy, from the earliest ages down to the present time. The annals of physical science do not, perhaps, furnish a more interesting picture of gradual advancement towards perfection than that which exhibits the successive improvements effected in this department of astronomy-from the naked estimations of the Chaldean priests to the refined and complicated methods of observation practised by modern astronomers-from the gnomon and the clepsydra, in their most rudimentary forms, to the transit circle of the Greenwich Observatory, the pendulum

clock of the most improved construction, and the electro-magnetic recording apparatus of the American astronomers.

In this chapter I have given a somewhat detailed account of the Royal Observatory of Greenwich, from its origin down to the present time. The observations which have emanated from that noble establishment have proved of incalculable service to astronomical science. No other similar institution, whether of ancient or modern times, can compare with it in this respect. Its history affords an instructive lesson regarding the advantage to be derived from applying the resources of an observatory to some definite object, and maintaining that object in view with unswerving constancy of purpose. An uninterrupted succession of eminent astronomers, who have directed the labours of this establishment, have contributed to render it the storehouse from which the materials for determining the elements of Astronomical Science have been mainly derived in modern times. With the triumphs of the Theory of Gravitation its history is inseparably associated. During the early period of its existence, it had the glory of supplying Newton with a series of observations, which served as a valuable guide to him while engaged in threading his way through the intricacies of the lunar theory, and it has continued ever since to furnish almost exclusively the astronomical facts, by an appeal to which the successors of that illustrious geometer have been enabled to establish the accuracy of their theoretical results. The recent reduction of the entire mass of the Greenwich Observations of the Moon and Planets, extending from 1750 to 1830, under the superintendence of the present Astronomer Royal, is an achievement which, while in respect of vastness it has few parallels in the annals of science, at the same time forms one of the most valuable acquisitions which Astronomy has received during the present century.

As a fitting sequel to the subject above-mentioned, the nineteenth chapter contains a brief account of the labours of astronomers in the construction of Catalogues of Stars. It is impossible to exaggerate the importance of this department of Astronomical Science. The places of the stars constitute so many fundamental facts, upon which depend all exact conclusions relative to the movements of the planetary bodies. The labours connected with their determination afford ample scope for talents of the highest order; but it must be acknowledged that they offer little to captivate either the imagination or the intellect, while at the same time they demand the most arduous exercise of the attention, and the most unflinching perseverance. Despite these disadvantages, there have not been wanting numerous examples of astronomers who, disregarding the éclat which usually attends discovery, have devoted the best portion of their lives to the construction of a Catalogue of Stars. Lacaille, Piazzi, and Groombridge will be especially remembered in the annals of astronomy, as individuals who sacrificed their days and nights with unwearied assiduity to this object, cheered only by the consciousness of the advantages which posterity would derive from their labours, and

by the secret charm which a constant intercourse with nature never fails to yield.

The twentieth chapter contains the history of the Telescope. The researches of Van Swinden have recently contributed to throw much interesting light upon the original invention of that instrument.

The twenty-first chapter, which completes the work, is devoted to a condensed account of the progress of researches in Stellar Astronomy. The labours of modern enquirers in this department of Astronomical Science have led to some conclusions of a highly interesting and important nature. The existence of a sensible parallax in the fixed stars-a question which has occasioned much anxious investigation from the time of Copernicus down to the present day-has at length been definitively established in several instances by the labours of Bessel, Henderson, Struve, Peters, and Maclear. It is now ascertained beyond all doubt, that light, travelling at the rate of 192,000 miles in a second, would require three years and a half to traverse the space between one of the nearest of those luminaries and the earth! The motion of the solar system in space, is another of those sublime conclusions which have been established by the researches of modern astronomers. It appears from the labours of Sir William Herschel and his successors on this subject, that not only do the satellites move round the planets, and the planets round the Sun, but that the Sun, with his whole cortège of planets and satellites, is being constantly transported through space to a determinate point in the heavens, revolving, in all probability, round the centre of gravity of some vast system of suns, of which it forms one of the constituent members. Thus the farther the human mind is allowed to penetrate into the mechanism of the physical universe, the more overwhelming is the impression produced of the surpassing grandeur of its movements, and the more exalted is the conception formed of the Omnipotent Being who constantly presides over its countless arrangements.

To the student of Celestial Physics, the researches of astronomers on Double Stars offer a high degree of interest, inasmuch as they serve to demonstrate that the law of Gravitation, as announced by Newton, actually prevails in the mutual action of those remote bodies of the universe. The phenomena of Nebulæ excited little interest among astronomers until they attracted the attention of Sir William Herschel. The vast extent of that astronomer's observations of those objects, and the originality of his views on their physical constitution, had the effect of elevating them to a high degree of importance in sidereal astronomy. The subsequent labours of Sir John Herschel and Lord Rosse, in the same field of enquiry, have materially contributed to the advancement of our knowledge respecting those wonderful structures.

After a rapid view of the progress of research on the various subjects above mentioned, allusion is made to the labours of astronomers on the physical constitution of the Milky Way and the Distribution of the Stars in space. The chapter concludes with a brief account of the interesting

speculations of M. Struve on the Extinction of Light in its passage through

space.

In the prosecution of these labours I have generally endeavoured to elucidate the various facts of history by reference to the fundamental principles of astronomical science, adhering as closely as possible to the ordinary phraseology of language. Occasionally, however, the subject considered, does not naturally admit of concise elucidation, so that an adherence to this practice would have led to inconvenient-I had almost said interminable-digressions. On the other hand, to have omitted all allusion to such subjects would have been to sacrifice the principle of continuity which forms so essential an attribute of history, and to present the reader with an avowedly mutilated work. I have endeavoured to avoid these two extremes, by noticing every fact which seemed to constitute an essential link in the chain of historic exposition, but studiously aiming at conciseness in all those instances wherein explanation would be necessarily so prolix as to defeat its own object. This remark applies more particularly to the subjects relating to the Theory of Gravitation. With respect to the remaining portion of the work, it is to be hoped that no reader possessing an ordinary acquaintance with the elements of astronomical science can experience any difficulty in pursuing it through its details.

In a work demanding extensive research, and embracing a great variety of subjects, some of which are of a very abstruse nature, it is not pretended that imperfections may not be discovered. I may be permitted, however, to state, that it has not been without carefully consulting all the original authorities accesssible to me, and bestowing an attentive consideration upon each subject which it embraces, that I have ventured to submit this production to the judgment of the public. It is to be hoped that the accomplished reader will be enabled to discern in the following pages sufficient evidence of the justness of this statement, to induce him to regard with indulgence the shortcomings of the author in so far as his personal abilities are concerned.

It affords me sincere pleasure to have this opportunity of gratefully acknowledging my deep sense of obligation to Captain R. H. Manners, R.N., Secretary of the Royal Astronomical Society, whose kind encouragement and readiness in promoting my views I beneficially experienced on numerous occasions while engaged in preparing these sheets for the press.

London, March 2, 1852.

R. GRANT.