Just as master Tobias Tims, with vehement gesticulation, was mouthing and murdering the lines of poor Coleridge, a bevy of beauties from Cheapside landed from a hackney coach, to get a little head-trimming for Alderman Marrowfat's great din ner-party; and, as the master of the ceremonies was off at a tangent to place chairs, the rain still continuing, I unfurled my umbrella, on his door-steps, wished the eloquent pruner of mustachios a hearty good-by-and exit. MILNE'S ESSAY ON COMETS.* In At the present time, when considera- and Englefield, prodigious advances ble excitation has been wrought in the public mind by the fancied appearance of one comet, and the expectation of another, when a feverish anxiety and terror has pervaded many classes, t-this Essay on Comets will be read with interest and curiosity. The generality of publications connected with this branch of the science of astronomy, are either too popular, or the facts are wrapt about with a mantle of obscurity, and veiled in mysticism; so that none but the initiated, and those who revel in intricacies, can derive any pleasure or instruction from the perusal. A profound knowledge of the science will not be requisite to enter into Mr. Milne's interesting details and discussions; while those who possess comprehensive acquaintance with the subject will have no reason to complain that the subject is treated superficially the accuracy of its descriptions, the clearness of its reasonings, and elegance of its formulæ, will ensure it a favorable reception, alike from the general reader and the man of science. a Such a work was eminently wanted. Since the treatises of Halley, Pingré, have been made in ascertaining the nature of comets, owing, in a great degree, to the number of laborers in the field, the excellency of modern instruments, and the improvements in the methods of observing. The records of the particulars resulting from these advantages were scattered in different papers presented to learned societies, in periodicals, foreign ephemerides, and occasional tracts. availing himself of these resources, Mr. Milne has been judicious in selecting what is worth preserving, and bringing it to bear upon the subject on which he is treating. A work of this kind was not required for merely a scientific purpose-to gratify the philosopher : it was desirable with a view to dispel those remaining mists of superstition and vulgar prejudice which yet overspread a very large portion of society. Of this we have many recent instances, and those not altogether in the lower walks of life. That which is uncommon, and apparently against the course of nature, more powerfully strikes the senses and affects the passions, than the uniform yet sublime phenomena of the universe. The superficial observer, as his eye unconsciously wanders Essay on Comets, which gained the first of Dr. Fellowes's Prizes, proposed to those who had attended the University of Edinburgh within the last twelve years. By David Milne, A.M. F.R.S.E. 4to. pp. 189. Edinburgh, 1828, Black; London, Longman & Co. Scarcely a day has passed but references have been made in the public prints to a comet said to be seen in the E.N.E.; and in some journals, not only its appearance described, but also its course,-that it was traversing from the bright star in the head of the Ram to that in the head of Andromeda, which star it would eclipse in its progress. This fancied comet we stated some weeks since to be the nebula in the girdle of Andromeda, which has been known to have occupied the same place in the heavens from the earlier ages of astronomy, at least as far back as 905, A.D.; it is very visible to the naked eye. Venus, also, from its unusual brightness as a morning star, has been mistaken for a comet; and respecting the luminous arch visible on the 29th September last, a correspondent in a useful miscellany (Mechanic's Magazine, Oct. 11th) inquires, "If the comet of Encke were passing in a direction towards the sun, might not its tail present the above appearance ?"!! over the spacious vault of heaven, gemmed with splendid suns and worlds, sees nothing but sparkling points if his mind should be so long fixed as to observe that this mighty assemblage is moving round the glowing pole, Rolling along, like living cars Of light, for gods to journey by! he can rarely be brought to think of that admirable mechanism by which they pursue their circling way. But should the solar orb be obscured at mid-day by the interposition of the moon, and the fair face of nature be shrouded in awful darkness-should a splendid stream of mysterious light spread its arch across the sky-should a fiery meteor rush through the heavens-or a comet, like the spirit of a desolate world, shake far and wide its tremulous tresses,-terror and curiosity are at once excited to the full, and we hear of the fall of princes, the ruin of empires, and the dissolution of the globe itself. When one of these glorious strangers unexpectedly bursts upon the view, and appears amidst the wilderness of stars, with what different feelings is it contemplated! The gloomy ascetic will say it is the abode of the damned; others, that it indicates the death of the illustrious and noble: the comet of 1811 was considered as the baleful star of Napoleon-to forwarn the destruction of his armies; the burning of Moscow also followed this celestial omen. The farmer scowls at the comet, which parches his fields, or, as it may happen, that drowns his crops; while the votary of Bacchus, as he quaffs his wine, blesses the comet, which improves the vintage, producing wines concentrated as its nucleus, and brilliant as its tail. But not only direful effects were said to attend the appearance of these bodies, they were supposed to generate atmospherical changes, affecting the productions of the earth and the ani mal kingdom; and this was the opinion as recently as during the appearance of the comet of 1811; it was noticed that the summer and autumn of 1811 were, over the whole of Europe, remarkable for long continued heat, and the cause was generally ascribed to the great comet which appeared during the course of that year. Hence connoisseurs in wines are still in the habit of distinguishing the claret made from the vintage of that year by the appellation of the "comet wine," on account of the effect which this luminary was supposed to have had in maturing the vintage. But enough of so deplorable an example of astrological faith, more worthy of the darker ages, than of a country and times so enlightened as ours. That division of the Essay which treats of the motion of comets through the system, will, we suspect, be read with considerable pleasure by those who desire to see the most intricate investigations of astronomy in their most simple forms;-we mean the calculations of a comet's orbit on the parabolic hypothesis, which is illustrated by determining the elements of the comet of 1826. In perusing this, we are furnished with a striking proof of the advances made in determining the periods of comets, by contrasting the ideas entertained by Halley, who termed that which bears his name, whose period is about seventy-five years, "the Mercury of comets," on account of its supposed short revolution, when compared with many others ;-what would he have said of the comet of Encke,* whose period is only 1203 days, and the comet of Gambart, whose orbit is completed in not more than six years and three quarters, or 2,461 days! But we dare assert, that the part of this Essay which will be most interesting to the general reader, will be that which treats of the collision of this earth with a comet. This is the comet so frequently referred to by continental philosophers, which at its next approach (in 1832) will pass the earth's orbit at the distance of about 14,000 leagues, but at a period when the earth will be in a different part of its orbit, and therefore no mutual attraction can by any possibility take place. "It was apprehended by many astronomers, that if a comet were to approach the earth, within a short distance of its surface, the attraction of the comet might be sufficient to elevate the ocean to a prodigious height, and thus occasion all the horrors of a deluge. La Lande computed, that were a comet of the size of the earth to come within 13,000 leagues, or about five or six times nearer than the moon, the waters of the earth would be raised 2000 toises above their ordinary level, and thus inundate all the continents of the world.' Such would undoubtedly be the effect of the mere proximity of the comet; but, as Du Sejour very justly remarks, this result is materially modified by several circumstances. La Lande's calculation is founded on the supposition, that the comet remains vertical over the same part of the earth, till the full effect of its attraction is produced. Now, Du Sejour shows in the most satisfactory manner, that, supposing the ocean to have a uniform depth of a league, nearly eleven hours must elapse before the inertia of the waters could be overcome; if the depth be supposed two leagues, eight hours and a quarter would be necessary. But, 1st, The comet cannot remain beyond a very short period over the same spot, on account both of its own progressive motion and the rotation of the earth. 2d, The comet would soon have removed to so great a distance, as to lose all its power of attraction. 3d, The waters of the ocean are not spread uniformly over the surface of the globe; and this is a circumstance which, as in the Mediterranean and other inland seas, diminishes very considerably the elevation of the tides. But, along with these considerations, it is essential also to remember the small mass which characterises the generality of comets. La Place, as was already stated, showed that the mass of the comet of 1770, one of the largest ever observed, could not have amounted to 1-5000th part of the mass of the earth but assuming that its mass was 45 ATHENEUM, VOL. 1, 3d series. even equal to this, what is the actual effect which its attraction could have produced on the ocean, in comparison with the moon's influence! The power of attraction, it is well known, is proportional to the mass; so that if we assume the comet of 1770 to have had a power of attraction equal to 1-66.6th part of the moon's, and modify this according to the law established by Newton, that the effect increases in the inverse triplicate ratio of the distance, we find, that in order to produce only the same elevation of the tides as the moon does, the comet must be (66.6), or about four times nearer to the earth than the moon. But at so short a distance, and possessing, therefore, so great an angular velocity, the comet would have passed by long before any such effects could have taken place. "By proximity alone, comets are almost wholly incapable of affecting either the movement of the planets, or the system of things upon their surface. But the case is very different, on the supposition of actual contact: for one of those circumstances which would be the chief means of counteracting the comet's influence in approaching a planet, viz. the rapidity of its motion, would serve, by the momentum, to give great effect to a collision. Still it must be observed, that, though this occurrence will necessarily be attended with far more alarming consequences, it is one of which the risk is infinitely less than a mere approach. For, in order that the collision should happen, it is requisite, first, that the radius vector of the comet be exactly equal to the planet's distance from the sun; secondly, that the comet be in the plane of the planet's orbit; and thirdly, that the longitude of its ascending or descending node be the heliocentric longitude of the planet. When, therefore, we consider the improbability that all these conditions should be simultaneously fulfilled, and add to this circumstance, the immensity of the celestial spaces through which the orbits of comets extend, it will at once appear how whose nucleus, if Herschel's estimate unlikely it is that such an occurrence should take place in the succession of many ages. But though the probability of such a collision is extremely small, we see that it is perfectly possible in itself; whilst the amount of that probability may be greatly increased by lapse of time. Let us now, therefore, shortly attend to the consequences which might ensue from such an event. It is evident that much will depend on the direction of the comet's course at the time of its encountering a planet. If both be moving towards the same quarter of the heavens, each will glide off from the surface of the other, and no very material changes will be produced, either on their movements or on their physical constitution. But should the directions of their respective courses be exactly opposite when the concurrence takes place, (a case, however, which it is easy to see can happen only with retrograde comets,) the consequences would necessarily be far more serious and permanent. It is true, that in general comets are of very inconsiderable magnitude; but the deficiency of mass is amply compensated by the prodigious momentum, by means of which a planet might be impeded, or even altogether arrested, in its orbit. If, for instance, a retrograde comet, moving at the rate of 1,734,000 feet per second, should in this manner meet the earth, assuming the earth's velocity at the time to be 102,000 feet per second, the shock would have the effect of at once destroying the progressive motion of both bodies, and causing them to fall to the sun, were the comet's mass only about one-seventeenth of the earth's, or four times that of the moon. It is true, we have no very authentic records of many comets of such a size having been observed; though, even if there were none at all, the fact would afford an illustration of our limited knowledge, rather than a proof of the non-existence of such bodies in the system. But even in our own times a comet has appeared, be correct, exceeded the moon in diameter, and which, if it had chanced to strike this body in a particular direction, would most infallibly have caused it to descend to the earth's surface. Seeing, then, that the collision of a comet and planet is an event lying within the verge of possibility, have we any reason to suppose that it is one which has ever happened? This question we can answer, only by examining the movements and constitution of the planets as they at present exist, and tracing back the circumstances now characterising both to those causes by which they seem to have been produced." Mr. Milne has some curious ideas on comets considered as habitable bodies, and very ingeniously shows that there is no absurdity in the supposition, but that it is perfectly agreeable with the economy of the uni verse. "If we estimate the intelligence of beings by the knowledge which their place in the universe is fitted to impart, we are compelled to regard the cometary inhabitants as of an order even superior to the creatures of the earth. When, for example, they find themselves passing through the midst of the satellites, those small bodies which we can scarcely discern with telescopes-or when they are brought so close to the planet Saturn that they can examine the wonderful phenomenon of his rings even with the naked eye-or when, at the perihelion passage, they are able to observe every thing on the surface of the sun, that great luminary, the mysterious source of life, and light, and energy, to the system,-what spectacles of delightful contemplation must they enjoy, and what means of attaining an acquaintance with the works of nature, infinitely greater than any which we shall ever command! Traversing, as they do, the whole extent of that system of which the earth forms so insignificant a member, and directing their course far beyond its known limits into those regions of space, whose dark and un The Butterfly. fathomable nature it will forever baffle human penetration to explore, the beings who have their abode on comets must be familiar with many important truths of which we can obtain only a few casual glimpses, and witness such glorious and sublime displays of the manifold wonders of creation, as must afford to them the noblest conceptions of that Almighty Being by whose wisdom they were constructed, and by whose power they are still sustained." Mr. Milne, in his advertisement, indirectly apologises for not having studied elegance of composition, or glossed over his Essay with the gay coloring of fancy; but the following specimen will sufficiently prove that he can not only write with accuracy as to related facts, and with perspicuity as to mathematical reasoning, but also with considerable beauty and eloquence. "When we contemplate the astonishing discoveries which this same pitiful creature, man, has effected, concerning the movements and origin of the heavenly bodies, as well as the extent and constitution of the planetary system, we are lost in wonder and admiration. Darting his feeble vision from the surface of his own globe, by means of the telescope he directs his inquiring eye to the farthest limits of creation; he examines other worlds moving in their various courses, at almost immeasurable distances from his own; he is able to discover the peculiarities of their orbits, and even to obtain intelligence respecting their physical structure. Those other bodies, the comets, which withdraw themselves far be 363 yond the reach of perception, are not THE Butterfly was a gentleman THE BUTTERFLY. His lady-wife was a poor glow-worm, And he roved in the sunshine all day long, She loved him better than all the world, In his scarlet and purple suit: And he left his lady-wife at home In her own secluded bower; Whilst he, like a bachelor, flirted about Though little he cared for her. Unheeded she pass'd the day--she knew But when night came on, she lighted her lamp |