possess this power of obstruction. It results, therefore, that the faintness which small stars occasionally exhibit, when perceived through any part of the substance of a comet, may, and in all probability does, to some extent, arise from the resistance which the cometic particles oppose to the rays of light proceeding from the star to the observer *.

When the flimsy nature of the substance of a comet is taken into consideration, it is difficult to conceive how such a body is not entirely dissipated in space by the enormous heat of the sun during its passage of the perihelion. Mention has already been made of Newton having calculated that the great comet of 1680, when it arrived at its least distance from the sun, was subjected to a heat 2000 times greater than that of red-hot iron. He considered that the circumstance of a comet being able to maintain its existence after passing through such a terrible ordeal, formed an irresistible argument in favour of its being a solid body. Laplace, availing himself of Black's beautiful discovery of latent heat, shewed that the durability of the existence of a comet might be accounted for, without having recourse to a principle which, to say the least respecting it, did not receive any support from observation. It was established by the eminent philosopher just cited, that when a body is in the course of passing from the liquid to the gaseous state, the particles, as they become successively volatilized, abstract from the body a large quantity of caloric which continues insensible to the thermometer. Laplace supposed that the heat thus carried off by the volatilized particles of the comet during its passage of the perihelion, would serve to moderate the temperature of the more condensed portion; and conversely, the heat given out by the same particles, in the course of their return to the liquid state, would have the effect of counteracting the intense cold to which the comet would be exposed in the more distant parts of its orbit ‡.

Whether a comet be composed of a partially solid substance, or whether it consist of a mere collection of vapours, is a question which has not yet been resolved to the satisfaction of astronomers; but one thing is certain, that the masses of comets must be very small. This was strikingly evinced in the case of Lexell's comet by its passage through the middle of the system of Jupiter's satellites, in the year 1779, without occasioning the slightest perceptible derangement in the motion of either of those bodies.

It seems difficult to account for the extreme faintness of the stars seen by Sir William Herschel through the comet of 1807, without supposing it to have been in some degree produced by the interposition of the cometic substance. On the other hand, Sir John Herschel has remarked that, although innumerable stars of all magnitudes, from the ninth downwards, were seen by him through the substance of Halley's comet, there never appeared the least ground for presuming any extinction of their light in traversing it. "Very small stars were, indeed, obliterated," says that eminent astronomer, "as they would have been by an equal illumination of the field of view; but in no case to a greater extent than they would have been by so much lamp-light, artificially introduced. (Results of Ast. Obs. at the Cape, p. 401.)

+ As this result of Newton's is often cited, it may not be out of place briefly to state the grounds upon which he established it. When the comet was in perihelion, on the 8th of December, its distance from the centre of the sun was to the earth's distance as 6 to 1000. Now, since the intensity of the sun's heat is reciprocally as the square of the distance from his centre, it follows that the sun's heat on the comet was to the heat of the summer sun as 1,000,000 to 36, or as 28,000 to 1. But he also found by experiment that the heat of boiling water is about three times greater than the heat which dry earth acquires from the summer sun; and he moreover conjectured that red-hot iron is about three or four times hotter than boiling water. His final conclusion consequently was that the comet must have been subjected to a heat 2000 times greater than that of red-hot iron.

Système du Monde, tome i., book ii., chap. v.

The question with respect to the end which comets are designed to serve in the economy of creation, appears to be involved in a degree of obscurity greater even than that which surrounds any other enquiry connected with these mysterious bodies. Newton asserted that all those comets which descend so low as to come within the solar atmosphere, would suffer a retardation of their motion on each occasion of their passage through their perihelia, and being, in consequence, less capable of resisting the attraction of the sun, would gradually approach that body until they ultimately fell upon his surface. Generalising this idea, he supposed that the fixed stars might be occasionally recruited by the falling of comets into them, and that the conflagration hence arising might account for those temporary stars which, at different times, have appeared with great splendour in the heavens. With respect to the tails of comets he was of opinion that after being dissipated in space they were absorbed by the planets, and entering into a multitude of chemical combinations with other substances, tended thereby to repair the waste of fluids occasioned by the evaporating influence of the sun. It would, perhaps, be as difficult to disprove these surmises as to demonstrate their truth, for in fact, they can only be regarded as mere sallies of the imagination into regions of thought, beyond the reach of legitimate reasoning. The speculations of succeeding astronomers on this subject do not lead to conclusions of a more satisfactory kind than those above hinted at. Sir William Herschel was of opinion that a comet on the occasion of each perihelion passage acquires a more perfect state of condensation in consequence of the action of the solar heat upon the nebulous matter of which it is partially composed. This hypothesis pointed out to the probability of a comet eventually acquiring the consistency of a solid body and assimilating itself in all respects to a planet. A similar view of the ultimate state of comets was adopted by Laplace, who remarked that the comet of 1759 was the only one which had hitherto exhibited any indications of having arrived at a fixed condition. He was probably led to this conclusion by a comparison of the recorded apparitions of the comet, from which it would seem that it had been diminishing in splendour on the occasion of each return to perihelion, until at length, in 1759 it almost ceased to exhibit the more striking peculiarities of a cometary body. Much of the awful magnificence of the comet on the occasion of its apparition in 1456 is doubtless attributable to the effect upon the imagination of a phenomenon that was universally regarded with feelings of terror, as a visible manifestation of divine displeasure; but it is an indisputable fact that the comet was a much more conspicuous object in 1607, than it was in 1684 or 1759. Thus, in 1607, Kepler distinctly perceived the tail with the naked eye, thirty days before the comet's arrival in perihelion *. In 1682 the comet, even for some time after it became visble to the naked eye, did not exhibit any vestige of a tail, for Cassini has remarked that its disk was as round, as well defined, and as devoid of nebulosity as that of the planet Jupitert. Perhaps the circumstance of its appearing so soon after the great comet of 1680 may have caused its cometic features to be in some degree overlooked by astronomers. This could not be said on the occasion of the return of the comet in 1759, for although Messier carefully

The comet with its tail was seen at Prague by many persons as well as Kepler on the 26th of September (De Cometis, p. 25). According to Halley it passed its perihelion on the 26th of October.

+ Mém. Acad. des Sciences, 1699, p. 39.

observed it with a telescope twenty-six days before the passage of the perihelion, he was unable to discern the slightest trace of a tail. So far, therefore, the remark of Laplace appears to be supported by observation. Unfortunately, however, the comet during its last apparition exhibited a decidedly less planetary aspect than it did in 1759; for on the 12th of October, 1835, the tail was already visible to the naked eye as a very conspicuous object, although the passage of the perihelion did not take place until 35 days afterwards.

The foregoing account of speculations on the physical constitution of comets may serve to shew how much yet remains to be done in this interesting department of astronomy. It is clear that a more extensive collection of facts than that at present in the possession of astronomers, must be formed by a long course of accurate observation; and that more mature views of the great agents of nature must be arrived at by an assiduous cultivation of the various branches of physical science, before any hopes can be entertained of coming to a definitive conclusion respecting the more essential properties of these mysterious bodies, or the purposes they are designed to accomplish in the economy of the material universe.

CHAPTER XVI.

Importance of Facts in the Cultivation of Physics.-Astronomy a Science of Observation. Inequalities which affect the apparent positions of the Celestial Bodies.-Precession. Its Discovery by Hipparchus. - Researches of Modern Astronomers on its Value. - Bessel.-Peters.-Otto Struve. - Refraction. Its effect upon the Place of a Celestial Body first remarked by Ptolemy.-Opinion of Tycho Brahé respecting its Nature. The first Theory of Refraction due to Cassini.-His Table of Refractions.— Newton. His Correspondence with Flamstead on the subject of Refraction.-Formula of Bradley.-French Tables of Refraction.-Researches of Bessel. -- Aberration. -Its discovery by Bradley.-Modern Determinations of its Value. -Nutation discovered by Bradley.-Its most Approved Value.—Researches on Parallax.— Methods for facilitating the Reduction of Observations.-Method of Bessel.-Physical Causes which more especially affect the Aspect of the Celestial Bodies.—Diffraction.—Irradiation.

Ir does not require a profound aquaintance with the history of any branch of physical science, to arrive at the conviction that its advancement has been invariably effected by reasoning upon facts whose existence had been already established either by observation or experiment. Astronomy is essentially a science of observation. Even in the earliest stages of its progress some interesting results were deduced, by simply noting the periodical recurrence of the more obvious phenomena. It was by pursuing a process of this sort that the Chaldeans succeeded in obtaining rude approximations to the times of revolution of the sun and moon, and in predicting the occurrence of lunar eclipses. The earlier philosophers of Greece, misled by erroneous views with respect to the mode of discovering truth, imagined that it would be inconsistent with the dignity of the human mind, to recognise any alliance between the lofty speculations of abstract science and the monotonous task of observation. It followed, as a necessary consequence, that during many years of Grecian history, not

even excepting the palmy days of Athenian civilisation, no progress was made in the study of astronomy. It was only when Alexandria became the capital of the civilised world, and learning in all its departments was liberally patronised by the Ptolemies, that the phenomena of the heavens were observed with regularity and care by the aid of instruments invented for that express purpose. Accordingly, astronomy, considered as a science of strict calculation, was during this period established on a durable basis. It cannot be asserted, indeed, that even yet the metaphysical notions of the speculative philosophers had been wholly banished from the science. The Aristotelian dogmas respecting the essential nature of the celestial movements were still regarded as indisputable axioms, between which on the one band, and nature on the other, a sort of compromise was effected by means of the famous mechanism of epicycles. So long, indeed, as the discordances between its results and the actual phenomena of the heavens did not exceed the probable errors of observation, this system, however complicated, might fairly be regarded as a legitimate representation of established facts. It was only when the advanced state of practical astronomy allowed the repudiation of discordances of such magnitude, that the arbitrary creatious of the human mind, and the immutable laws of the physical universe, might be said to have come into direct collision. The triumphant establishment of the true system of nature by the immortal Kepler, led to the complete emancipation of astronomical science from the thraldom of the Schools, and its subsequent history has in consequence been one of uninterrupted progress down to the present day.

Since the laws which regulate the movements of the celestial bodies constitute the principal subject of research in the study of astronomy, it is manifest that the establishment of a series of facts relating to their apparent positions, forms an indispensable preliminary to all such enquiries. Accordingly, the sun, moon, and planets have in all ages been carefully observed with this object in view; and all the resources of mechanical skill, as well as the most profound investigations of physical science, have been applied towards assuring the accuracy of the results. The stars, too, have been observed with equal care, not merely on their own account, but also because they form fixed points, to which the positions of the various bodies of the solar system may be on all occasions referred.

But the simple determination of the apparent positions of the celestial bodies does not suffice to produce results immediately available towards the purposes of astronomy. Certain inequalities of small, but variable magnitude, affect the position of every celestial body, the values of which must be carefully ascertained for each observation, in order to arrive at a knowledge of the mean position of the body, which alone can be employed in forming the basis of ulterior research. These minute displacements arise from the combined operation of various distinct principles, the investigation of the laws of which forms one of the most important departments of astronomical science. When considered with respect to their origin, they admit of a threefold division. In the first place there are inequalities which depend upon the principle of gravitation; such are the phenomena of Precession and Nutation. The second class of inequalities includes those which are explicable by reference to the properties of light; such are the phenomena of Refraction and Aberration. Lastly, there is the displacement occasioned by Parallax.

It appears from the foregoing remarks that every celestial body is subject to an apparent displacement arising from the combined influence of

five distinct inequalities, of which four derive their origin from physical causes; while, on the other hand, the fifth depends upon considerations of a purely mathematical nature. The apparent position of a body is reduced to the mean by applying to it, with an opposite sign, the numerical value of each inequality, corresponding to the time of observation. A brief account of the researches of astronomers in connexion with each of these five inequalities or corrections, as they are technically termed, may, perhaps, not prove uninteresting to the reader. This will be best effected by alluding to each correction according to the order of its discovery.

Of the various inequalities which require to be taken into account in reducing the apparent position of a heavenly body to its mean position, the one which first became known to mankind is the increase of longitude, arising from a slow regression of the equinoctial points upon the plane of the ecliptic. As this constant shifting of the intersection of the ecliptic and equator causes the annual arrival of the sun in either of the equinoxes to be a little earlier than it would otherwise be, it has in consequence been denominated "the Precession of the Equinoxes." The discovery of this apparent movement is due to Hipparchus, who arrived at it about the year 125, A. c., by a comparison of his own observations with those of Timocharis, made about 170 years earlier. Its existence was afterwards established beyond doubt by Ptolemy, between whom and Hipparchus there elapsed an interval of nearly 300 years. It has been already mentioned that Copernicus was the first who gave the true explanation of this phenomenon. The discovery of its physical cause by Newton, and the researches of his successors on its laws, have also been briefly noticed. It only remains to give some account of the successive determinations of its quantitative value by astronomers.

The earliest statement of the value of precession is to be found in the Syntaxis. Ptolemy mentions, in the seventh chapter of that work, that having observed several bright stars in the zodiac, he found that while their relative positions were the same as in the days of Hipparchus, they had all increased in longitude to the extent of 2° 40′ during the interval that elapsed between that astronomer and himself. He hence inferred that the increase of longitude amounted to 1° in 100 years, which implies an annual precession of 36"; he moreover stated that Hipparchus had arrived at the same result. This was a very erroneous determination, for, according to the researches of modern astronomers, the annual amount of precession is a little in excess of 50". The interval between Hipparchus and Ptolemy comprehended a period of 267 years, so that the total increase of longitude must in reality have amounted to 3° 37', a quantity greater nearly by 1° than that assigned by Ptolemy. As the discordance seems too great to be accounted for by errors of observation, except by adopting an extravagant supposition with respect to their probable magnitude, many eminent astronomers have come to the conclusion that Ptolemy made no observations at all; that in fact his catalogue of the stars is no other than the catalogue of Hipparchus reduced to the epoch of 137 A.D., by increasing all the longitudes to the extent of 2° 40'. Unfortunately there are circumstances which strongly tend to justify this serious charge. Delambre compared together the longitudes of 312 stars as assigned by Ptolemy with the longitudes of the same stars inserted in Flamstead's catalogue, and supposing the interval between these two astronomers to comprehend a period of 1553 years, he hence deduced 52".4 for the annual value of precession. This result exceeds the true value by rather more