no more; plants and flowers would be unable to circulate their juices; and man himself would sink down to the level of a slow-moving quadruped like the sloth. There is thus with us an express adaptation of the varieties of living existence to the magnitude and circumstances of our globe; and it is philosophical to accept this fact as a guarantee that a similar adjustment obtains throughout the system with which we are connected. And whence this adjustment? Whence this marvellous proportioning of the power of the humble crocus peeping above the snow, and the magnificent rein-deer bounding across it, to the earth's volume and mass, so that the force of gravity is not too strong in the one case for the development of the flower, nor muscular energy too great in the other for the purposes of the animal? Epicurus, upon reading the well-known lines of Hesiod in his youth "Eldest of beings, Chaos first arose, Thence Earth wide-stretch'd, the steadfast seat of all is said to have proposed the natural question to his preceptor, "And Chaos whence?" The philosopher originated a theory in his riper years which assigned the creation to a fortuitous concourse of atoms; but how natural the inquiry, overlooked by the theory, "And atoms whence?" A sober understanding will not stop short of recognising a presiding Providence in adapting the economy of terrestrial life to terrestrial conditions ; and even so are we warranted, from the evidence of what is near, to look upon the remote, as the scene of similar adaptations, the operation of the same First Cause. ous. In regarding the planetary worlds as the abodes of sentient life and of forms of existence kindred to those which occupy the carth, we are in advance of what is written, or what observation detects, but not beyond what the sobrieties of reason will justify. It may be hard to imagine how life can be sustained under the apparent heat of Mercury, or amid the seeming cold, the tremendous storms, and rapid atmospheric changes of Jupiter. But, ignorant of facts, a parallel difficulty would be a stumbling-block to us, in relation to our own planet, when we consider the high temperature of its equatorial regions and the intense cold of its polar circles. Yet we have great families of men and animals in each extreme. We meet with human life upon the sultry plains of Delhi, and on the ice-bound shores of Greenland; and where the citron, the myrtle, and the palm will not flourish, the pines, the mosses, and the lichens grow. It is impossible to naturalise the elk in England, owing to its warmth; and turn the giraffe adrift, and how long would it survive the chill of the climate? Yet each animal, in circumstances to which it is adapted, is stately and vigorAll the planets are plainly of one family as to their physical character, their general configuration, their motions of revolution and rotation, and the alternation of day and night; and these are resemblances which may reasonably lead us to suspect other analogies. The fact is also clear, of Mercury, Venus, Mars, Jupiter, and Saturn being surrounded with atmospheres ;-a constitution which strongly indicates their occupancy with some varieties of organised being. We know, in the case of our own globe, the important uses of its atmosphere in maintaining animal life, transmitting sound and light, and in advancing the arts which tend to civilise society. Without such a gaseous envelope, bound inseparably around the earth, its partner in all its motions, yet no emanation from it, but a separate element, the ear would have no office to perform, the tongue would be speechless, and the service of the eye be greatly abridged. The song of birds, the hymns of religion, the eloquence of senates, and the utterance of relative kindness would perish. The fiercest waves would dash in sullen silence upon the strand; and mankind would have no medium of inter-communication beyond that of sign and gesture. We may well believe, therefore, that our world has been furnished with this elastic and essential apparatus, in order to adapt it for the reception of animal existence and intelligent inhabitants; and the inference is just, that a similar arrangement distinguishing other planets, points to the same destination. It is a possible conception,- but we should smile at the credulity of the man who believed it real, that a fleet of ships navigating the ocean, with sails unfurled and pennons flying, did so without a cargo in the hold, a crew on board, or an object in view. CHAPTER V. COMETS. F all the celestial objects which have arrested the attention of mankind, none have excited such general and lively apprehension as those upon the consideration of which we now enter. Undoubtedly their sudden appearance, rapid movements, and occasionally extraordinary aspect, were calculated to awaken terror in ages of ignorance and superstition, and to originate the wild conjectures that are on record respecting their character and office. The Romans regarded a comet which was seen in the year 44 before our era as a celestial chariot conveying the soul of Cæsar, who had been assassinated a short time before its advent, to the skies. Cometary bodies have been deemed the vehicles in which departed spirits are shipped by their guardian angels for the realms of Paradise; and on the other hand, they have been viewed as the active agents of natural and moral evil upon the surface of the earth, and been formally consigned to ecclesiastics for excommunication and cursing. A volume of no inconsiderable dimensions might be compiled, and not without interest, from the accounts of old chronicles respecting their appearances, registering the quaintly expressed opinions of the chroniclers concerning them, the terrestrial events they have tacked to them as effects to a cause, and the deportment to which men have been moved by the apparition of "the blazing star Threat'ning the world with famine, plague, and war: To princes, death; to kingdoms, many crosses; To all estates, inevitable losses; To herdsmen, rot; to ploughmen, hapless seasons; To sailors, storms; to citics, civil treasons." We have the word comet from the Greek xóun, or hair, a title which had its origin in the hairy appearance often exhibited, a nebulosity, haze, or kind of luminous vapour, being one of the characteristics of these bodies. Their general features are a definite point or nucleus a nebulous light surrounding the nucleus, the hair, called by the French chevelure-and a luminous train preceding or following the nucleus. Milton refers to one of these attributes in a passage which countenances the popular superstition : "Satan stood Unterrified, and like a comet burned, Anciently, when the train preceded the nucleus, as is the case when a comet has passed its perihelion, and recedes from the sun, it was called the beard, being only termed the tail when seen following the nucleus as the sun is approached. This distinction has disappeared from all modern astronomical works, and the latter name is given to the appendage, whatever its apparent position. Neither this luminous attendant, the tail, nor the nucleus, are now considered essential cometary elements, but all bodies are classed as comets which have a motion of their own, and describe orbits of an extremely elongated form. There are several plain points of difference between comets and planets. The planets move in the same direction from west to east, which is astronomically called direct motion; but the movements of comets are often from east to west, or retrograde. The orbits of all the planets are confined to a zone of no great breadth on either side of the ecliptic; but the paths of comets cut the ecliptic in every direction, some being even perpendicular to it, traversing the heavens in all parts. The contrast is striking likewise between the forms of their respective orbits. A hoop will with no great inaccuracy represent the courses of the planets, but cometary paths are of every possible eccentricity, both elongated ellipses and open curves, as parabolas or hyperbolas. Only one end of the ellipse lies within the limits of the system, in the case of the great majority of comets with shut orbits. They only visit our gaze therefore during one part of their course, and that a very small part, travelling during the rest of their journey far beyond the range of the most distant planet, into spaces inaccessible to our sight. Those which describe parabolas and hyperbolas are casual vistors only; and depart to return no more. Planetary configuration is also uniformly globular, but the external appearances of comets exhibit great diversities of form, from that of an irregular wisp of cloud to a simple spherical luminosity, or a strongly-defined scimitar-shaped aspect. Most of the ancients, following Aristotle, regarded comets simply as meteors born and perishing in the atmosphere of the earth. Seneca, however, clearly classed them with the enduring realities of nature, having a definite path, and not wandering uncertainly through a transient existence: "I cannot believe," he observes, "that a comet is a fire suddenly kindled, but that it ought to be ranked among the eternal works of nature; it has its proper place, and is not easily moved from thence; it goes its course, and is not extinguished, but runs off from us ;" and in a passage already quoted, he anticipates the arrival of a Newton or Halley to determine their orbits, and the laws of their motions. Tycho Brahe took the initial step in the path of true discovery by assigning them a place out of the terrestrial atmosphere. By careful observation of the comet of 1577 he proved its extralunar position in space. It yielded no sensible diurnal parallax, and was therefore beyond the region of the moon. Hevelius next ascertained the concavity of the orbits of comets, which Keppler had supposed to be straight lines. Newton succeeded in demonstrating that comets are guided in their movements by the same principle as that which controls the planets in their orbits, as the law of gravitation admits of revolving bodies describing any one of the conic sections, or the four curves, the circle, ellipse, parabola, and hyperbola. Halley finally, after a laborious comparison of elements, arrived at a measurable ellipse as the orbit of one of these bodies; and predicted the periodic return of the object, which has twice appeared at the time appointed to verify his conclusion. The diagram represents a part of the path of one of the long period comets, that of 1680, obviously but a very small part, as it was described in little better than two months, and the periodic time is supposed to be not less than five hundred years. The direction of the luminous train or tail is shown, the frequent attendant of cometary bodies. This is nearly always away from the sun, frequently assuming a curved form. It increases in length with its proximity to the solar body, but does not acquire its greatest extent until after the perihelion or the point nearest to the sun is passed. If we regard the train as vaporisation produced by the intense heat to which the body of the comet is exposed upon approaching the sun, this accounts for its increasing length and greatest extent after the perihelion, just as it is after the summer solstice that the earth attains its highest temperature, although its daily supply Jan.25 Jan. 5. 1681 ORBIT OF THE EARTH Nov. 171680 Dec. 29 Nov,21 Dec 21 Nov,25 Dec12 SUN of solar influence is then actually diminishing. The comet appears in the diagram at its perihelion passage merely for the sake of illustration, as in that part of its course it was completely lost in the solar blaze. In the other positions it was observed at the times stated by Cassini, Newton, Halley, and Flamstead. Cometary statisticians have compiled a record of between six and seven hundred appearances since the commencement of the Christian era. But little dependence can be placed upon this enumeration, as simple meteors and such phenomena as new stars were confounded in former times with true comets, and instances of the re-appearance of the same body are no doubt included in the return. In about 200 instances, the orbits have been ascertained with more or less certainty. Of this number, forty appear to have described ellipses; seven hyperbolas; and a hundred and fifty parabolas. The ellipse is an oval which admits of every possible degree of eccentricity, or deviation from the circle; the hyperbola is an open curve, the branches of which may be considered straight lines indefinitely divergent; the parabola is an open curve, the branches of which extend in a nearly parallel direction, never converging. Consequently the elliptic comets are regular members of the solar system, with periodic times, some of them never wandering beyond the planetary orbits, while the hyperbolic and parabolic comets appear within its limits, depart, and return no more, unless thrown by some disturbing force into a new path. Perihelion. The three features of nebulosity, nucleus, and tail, are usually assigned to cometary bodies, but many are destitute of the latter appendage, and also without any clearly defined nucleus. They appear as simple nebulosities, globular masses of vapour, having no central condensation, through which the feeblest of the stars readily shine. Herschel perceived a star of the sixth magnitude through the centre of the comet without nucleus of the year 1795; and a star of the eleventh magnitude was perfectly distinguished by Struve through the middle of one of the short-period comets. Others present a nucleus strongly defined, with surrounding nebulosity, the "horrid hair" of poetry. The vapoury envelope is dim towards the central point, but suddenly becomes luminous at some distance from it, so as to resemble a ring resting in equilibrium around a star, like the ring of Saturn. The cometary nuclei often shine with a light as vivacious as that of the planets, and exceed them in splendour upon nearing the sun. They vary considerably in their diameters, but are in general very small. The measurement of the diameters of five given by Arago range between thirty-three miles and three thousand two hundred. The external appearance of other comets exhibits the three features combined, and these are remarkable objects, occasionally presenting a terrific aspect. Immense spaces are sometimes covered by the luminous trains, or tails, as much as ninety or a hundred degrees; so that while the nucleus has been below the horizon, the train has reached the zenith, stretching through an extent of nearly a hundred and fifty millions of miles. The tails appear to stream from that part of the nucleus which is farthest from the sun, but seldom in the direction of a straight line joining the two bodies. They generally exhibit a sensible curvature, bending towards that region of the heavens last quitted by the comet, and cases have been observed in which they have formed a right angle with the nucleus. The figure represents their common form, the arrow showing the direction of the comet's motion, and the dotted line the direction of the sun. There is great enlargement in the breadth of the tail, as its distance from the nucleus increases; and an obscure stripe appears passing down the middle, which has suggested the hypothesis of a hollow luminous cone. It is obvious that whether comets shine by inherent light, or reflect the solar rays, if the train be a hollow cone, a much greater number of nebulous particles will be in the direction of the eye at the sides than at the centre, which will account for the interior dim stripe, and the exterior brightness. Comets, however, are by no means confined to one train each, but as many as six have been observed appended to the same nucleus. While these variations of form may be due in a measure to different velocities, it is clearly ascertained that the aspect of the same cometary body undergoes great changes in its period of revolution. In recognising two apparitions as appearances of the same body, after having accomplished its periodic time, astronomers do not depend upon the circumstances of shape, size, or brilliancy being similar, but upon the elements of the path being accordant. Towards the close of the year 1680, a comet, illustrious on account of its observers, and apparently formidable from its aspect, appeared within the visible limits of our system, and approximated nearer to its centre than any other, except the remarkable comet of 1843. It finally vanished from terrestrial gaze in the month of March 1681, and has not since been seen. The mind of Europe was profoundly impressed with the vast size, velocity, and form of this object, which engaged the accurate observation of Flamstead and Cassini, and the mathematical science of Bernouilli, Newton, and Halley. After its perihelion passage, its appearance, as seen from Paris and particularly from Constantinople, was most imposing. The train reached to the zenith when the nucleus had set below the horizon, coruscations attending the whole length of the luminosity, giving to the phenomenon the aspect of a wrathful messenger, and not that of a tranquil body pursuing a harmless course. The greatest length of the tail was computed to be 123 millions of miles, and in two days an extent of 60 millions of miles was emitted from the nucleus, Its average velocity was upwards of 800 thousand miles an hour. A traveller through our heavens, covering such a space, and rushing with such speed through the firmament, might well excite the astonishment of mankind. It must not be imagined that this rate of motion is its average orbital velocity. In obedience to the Keplerian law its pace slackens in receding from the sun. According to the computation of Newton, this body approached the sun within the 163rd part of the semidiameter of the earth's orbit, being rather more than half a million of miles from his centre, and not more than 144,000 apart from his surface. If the projectile force had been stopped, in three minutes it would have closed with his mass. In such a situation it must have been exposed to a temperature which in an instant would dissipate any substance with which we are acquainted. Newton calculated the body of the comet to have been heated to a degree two thousand times greater than that of red-hot iron. This comet is supposed to be identical with the one that appeared about the time of Cæsar's death, with that which was seen in the reign of Justinian in the year 531, and with another in the year 1106 in the reign of Henry II. Comparing these dates, we find, from before Christ 44 to 531 leaves a |