The comet of Donati, which presented so imposing a spectacle in our northern hemisphere in 1858, strongly revived the popular interest in such appearances. Few of the present generation had an opportunity of gazing on any comet of remarkable splendour and magnitude. Our fathers told us of the wonderful comet of 1811, spanning half the circuit of the heavens with its tail, and hanging over the earth for weeks together like a drawn sword; we listened, however, as to a legend of the olden time, and despaired of our seeing such sights in our day. The comet of 1858 burst upon our view, and proclaimed that there were still wonders in nature, surpassing any that the eye of man had yet seen. Notwithstanding the tendency of the old to exaggerate the wonders of their youth, the privileged few who remembered the comet of 1811, confessed that Donati's comet was altogether a finer object, though it did not stretch so far across the heavens.

Though comets served no other object than to awaken fresh interest in celestial phenomena, their visits to our regions of space would not be in vain. Such startling phenomena serve to dispel the indifference with which we are prone to gaze upon the unvarying page of the "Book of God," presented to us by the ordinary aspect of the heavens. It was a bright star in the sixteenth century suddenly bursting forth and as suddenly dying out, that gave to the world the great astronomer Tycho Brahe. This imposing object determined his tastes and fixed his future career. Mr. Bond, the distinguished American astronomer, who has just been gathered to his fathers, dates his astronomical career from the total eclipse of 1806. This sublime and awe-inspiring spectacle took possession of his mind to such a degree, that he gazed with ceaseless and inquiring wonder on the face of the heavens ; this eager prying into the depths of space, gave him, while yet a boy, almost supernatural power of vision with the naked eye. As a watchmaker's apprentice he could not command the aid of instruments, but, with the faith of genius, he believed that he would not be denied a sight of nature's secrets, if he made good use of the advantages within his reach. He employed various arts to increase the sensitiveness of his retina. He practised looking into dark wells, with his eyes shaded from the light, to catch a glimpse of the almost invisible objects at the bottom. By this means, his eye acquired such keenness of vision, that he was the first in the American continent to discover the great comet of 1811. Little did the young apprentice think that he was, in this way, most effectually qualifying himself for the management, as head of an observatory, of some of the finest instruments in the world. His finely trained eye enabled him, in after life, to make the independent discovery of the dark ring of Saturn, and of a new satellite belonging to the same system. His early mechanical training enabled him to devise and perfect one of the greatest improvements in observation in modern times, viz., the substitution, by means of electricity, of touch instead of hearing to mark the instant of any astronomical event. To him, also, are we indebted for the art of celestial photography, which promises to revolutionize the system of astronomical observations. May we not hope that Donati's comet, in like manner, may have aroused the dormant powers of many a young astronomer who may live to wring from nature her deepest and most wonderful secrets?

One great charm of the comet, both in a popular and scientific view, is the illustration of formative processes visibly going on in the grandest scale. What would not the geologist give to see illustrated, in a conveniently brief space, the molten globe and the subsequent induration ; and the various upheavals and submergences, which his theories require? Now, the comet exhibits corresponding organic changes in the course of a few days or weeks, and on a scale vastly greater than what our earth could exhibit. What is it that renders the sight of the moon's surface so disappointing ? Is it not that all is motionless? There are no moving clouds, no fluctuating seas, no fleeting snow-covering, no active volcanoes:—all is motionless and deathlike. Why does the surface of the sun attract such deep interest at this moment? Because we discover agencies working with intense activity. There are elements of change detected, and these are watched with the most anxious interest. In the moon, we see a frozen torrent; in the sun, we see the stream dashing on with impetuous speed.

Measured by this source of interest, the comet arrests our attention much more powerfully than even the sun, the changes being much more rapid and comprehensive. We see the whole orb transformed before our eyes. We are apt to think that the full-sized comet is merely the same as the cloudy speck which we descried some weeks before, and that the whole difference of aspect is due to difference of distance; just as the white speck on the distant horizon, differs from the frigate entering the harbour with all her sails set. This, however, is far from being the case : no doubt increasing proximity magnifies the object; but apart from this, there is a wonderfully rapid expansion and change of form. The comet at a distance, is the rose in bud; in the proximity of the sun, it is the rose in full blossom. In the latter position, there is an astonishing development in size and shape. Perhaps a more apt comparison would be to the sea anemone, in which we have, with the same substance, an alteration of bulk and form. The comet in its remote position, is the shrunken anemone; with its bright nucleus and flaming tail, it is the anemone with its tentacles expanded, and its brilliant colours displayed.

With the naked eye we can only see the bright petals of the flower coming forth from the calyx; we do not detect the formative processes going on within the germ. But just as with the microscope we can dissect a flower, and detect the most secret processes of nature in the germ, so can we, with the telescope, penetrate the nucleus of the comet, and discover the plan of evolution by which the tail of the comet springs from this centre. As we watch with interest the potter fashioning an artistic vase from a mass of clay, so do we witness with a higher interest the comet, with all its symmetry and beauty, emerging from a mass of cloud, fashioned by great laws which are only the hands of the Divine Artist, by which it is moulded into the requisite form.

The illustrative figure at the head of this article is intended to have only a typical significance. We have been kindly favoured by Mr. Cooper with views of Donati's comet, as seen through his great refractor at Markree Observatory. His admirable sketches represent different stages of the development of the comet. We have combined them, so as to give in one view a clear idea of the structure of the comet. We have added another envelope to bring out more fully the stratified character, and to comprehend the observations of M. Chagornac at the Observatory of Paris. The particles of which the comet is composed are never motionless; they are in a ceaseless flow, impelled by forces which communicate to them an inconceivable rapidity of motion. The deepest mystery hangs over the nature of these forces, but the phenomena, as witnessed with the aid of the telescope, are easily described. When a comet is first descried in the heavens, it appears as a faint star, with a haze around it. It is often impossible to tell by its mere physical aspect whether it is really a comet. To solve the doubt, the observer must wait a little and see whether it is in motion. This assures him that is not a new nebula, and he concludes that it must be a comet. He traces for a short way the curve described, and having got a sample, he readily deduces the shape, dimensions, and position of the whole orbit. He proceeds, just as one would do in cutting out figures in the sward of a garden. When a portion of a regular figure is given, it is easy to cut out the whole figure. In this way the astronomer can readily tell how near it will approach the sun, and at what time. As the comet approaches the sun, internal changes can be detected. The globular mass is drawn out, the nucleus or brightest point being at the end nearest the sun. The other end soon assumes the appearance of a tail.

The forms of most large comets conform to one general type. The tail is slightly convex on one side, and has very much the appearance of a scimitar, the nucleus representing the hand with which it is grasped. The Chinese call comets broom-stars; and the appearance of many is very much that of a broom, bent and worn with much use. The outline of the convex is generally better defined than that of the concave side. There is a comparatively dark band running lengthwise through the middle of the comet, so that they have often the appearance of two tails; the two bright streaks on each side of the tail being taken for two distinct tails.

This is the appearance to the naked eye. When the nucleus is watched with the telescope, very remarkable changes are witnessed. The luminous matter of the comet rushes forth from the nucleus as the stream of fiery particles issues from a rocket. They rush out towards the sun with great velocity, but soon they reach a certain point, when they are driven back to form the tail. The appearance is not unlike that of a lighted torch held against the wind. The tendency of the flame to rise is counteracted by the action of the wind, which drives it back. The most perfect illustration we have seen of the internal form and movement of the comet, was the playing of the fountain in the garden of the Tuileries, close to the Place de la Concorde. The water rose to a considerable height, and fell back in an umbrella form. There was a slight wind, and the jet was slightly curved along its length, as in the case of the comet. The windward side was also best defined, the loose spray being all thrown to the leeward side. The descending canopy being hollow, the outline of both side's was more condensed than the central part, the eye having to look through a greater mass of water. The luminous particles shoot forth from the nucleus just as the water spouts forth from the jet, and are thrown back to form a hollow envelope. The curvature of the comet, and the sharper definition of the convex side, are usually explained on the supposition that the comet is moving through a resisting medium, so that an effect similar to the influence of the wind on a jet-d'eau is produced.

In the case of Donati's comet, the telescope detected the formation of numerous envelopes, so that the head presented the appearance of distinct stratification. The forces brought into action, seemed to be intermittent, and this intermission is marked by the boundaries of the strata. There is no continuous flow in nature, and the play of a fountain gives a very good illustration of this. However steady the pressure of the water may be, the fountain plays by jerks, and not by a steady continuous stream; and the development of the comet illustrates this on a grand scale, the different envelopes being the result. What is the structure that results from this view of the comet's formation? It may be represented by a series of hollow cones packed the one within the other. The grocer's paper cones may serve as a familiar illustration, or a nest of flower-pots, the smallest being in the centre, and the others forming concentric envelopes of increasing size. This will give a striated appearance to the tail, and when there is a space between the envelopes, the successive intervals will appear comparatively dark, so that the comet may seem to have many tails. Almost all the perplexing shapes comets assume, may be explained on this assumption of hollow concentric cones.

The next inquiry is, How is this extraordinary development produced? What are the forces brought into play? The jet of luminous matter from the centre is naturally explained by the action of the sun's heat, for the action is energetic in proportion to the nearness of the comet to the sun. The grand difficulty is the throwing back of the matter into the tail. It is evident that there is some repulsive power, and a power capable of communicating an inconceivable velocity to the repelled particles. The usual way is to ascribe it to electric or magnetic action. It is ascertained that the sun and the moon, as well as the earth, are magnets; and the great probability is, that all the heavenly bodies are magnetic. This seemed to afford a plausible explanation of the apparently polar forces at play in the evolution of the comet. Another, however, has recently been started, which has, for the time, put all others in abeyance, and is in process of being tested by the French savans. We allude to the hypothesis of M. Faye, who was commissioned by government to undertake the expedition to Spain, to observe the total eclipse on the 18th July, but who, from a misunderstanding with Leverrier, has thrown up the appointment, which has now been conferred upon the latter. The hypothesis is, that there is a repulsive force radiated from the sun, as well as the attractive force of gravitation, and he holds that this is only a single case of a universal law of matter. He holds that every incandescent body exercises this repulsion, but that it is so feeble, that it can be detected, only in the case of matter of such extreme tenuity as that of comets. He supposes that the force emanates only from the surface of the sun, and not, like gravitation, from its mass. With this simple assumption, he endeavours to explain the formation and all the phenomena of comets. The envelopes are produced by the expansive power of the sun's heat, balanced by this special repelling power. The shortening of the period of the comet of Encke is also explained by the same means. He has agreed to submit the law to the test of experiment. It was objected, that if the incandescent surface of the sun repelled the rarefied matter of comets, the same thing should happen in the case of incandescent bodies on the surface of the earth. He at once accepted the consequence, and agreed to put the thing to the test, if matter rare enough could be found and made visible. The extremely rarefied air left in the vacuum of an air-pump, was taken as the test; and a heated plate was employed to repel this matter. The difficulty was to make the repulsion visible. This was effected by passing an electric discharge through the exhausted receiver. The air was made palpable by means of the stratified light produced by a powerful coil. Several distinguished physicists hold the point demonstrated, though, as yet, most withhold their assent.

If this new law be established, it will be the most important advance since the time of Newton. It will, indeed, dethrone gravitation from the sole sway which it has hitherto exercised. Newton's law has been repeatedly put to the test, and more than once, astronomers trembled lest the whole Newtonian structure should totter and fall; but gravitation with its simple ratio, has come forth triumphant from the severest ordeals. One trial more awaits it, and all the astronomers of European fame are ranged on one side or other of the question. The two principal parties are M. Hansen of Altona, and M. Delauny, and with one or other of these the various combatants side. The controversy raged for some time in the Academy of Sciences between Leverrier and Delauny, but gradually all the great astronomers of the day have been caught in the vortex. Adams is almost the only astronomer of note that stands by Delauny; Pontecoulant and Plana are leagued with Leverrier in support of the results of Hansen. Delauny has elaborated a theory, by which he shows that the moon's actual motion does not accord with the law of gravitation. Hansen, on the other hand, asserts that his theory is correct, as his tables, adopted by the British Government, accord completely with observation. The precise point is, what is termed, the acceleration of the moon's mean motion, one result of which is, that the moon, in the course of the year, nears the earth by about an inch. Yet on this inch, depends the fate of the Newtonian theory. Adams and his French associate, hold that their results point clearly to some other cause than the law of gravitation, and M. Faye indicates his new law of repulsion as the true cause to explain the residual phenomenon. Though the controversy has not assumed a national aspect, the temper displayed on both sides ill befits the dignity of science. The comet in the hand of Newton broke through and shivered the crystal spheres of antiquity, and annihilated the vortices of Descartes. Strange if it be destined also to shake the foundations of even his own system itself!

When Newton failed to show how his law could prevent the perturbations of the planets introducing confusion into the system, he suggested that the direct interference of the Almighty might be employed occasionally to rectify matters. He and the theologians of his day seemed to think that there must be a defect in the celestial machine, unless it was proved to be capable of lasting for ever, and great was the rejoicing when the stability of the system was established. But suppose there is a resisting medium, accounting for the destructive course of Encke's comet towards the sun, or that the law of gravitation requires to be modified by some new law, which may compromise the present order of things by introducing an element of decay, are we to conclude that there is a defect of wisdom in the constitution of the celestial machine? We might as reasonably deny the traces of wisdom in the structure of our bodies, because they have not the stamp of eternity. Theologians have been too much led away with the idea of the solar system being a rigid machine, with unvarying adjustments, instead of a single phase of the mighty evolutions of the material universe. There is nothing fixed and rigid in the material world except the laws by which the all-wise Ruler governs it. Every star in the heavens, and every molecule in the earth, is in incessant movement. There is a constant flow, and the wisdom of the Creator is seen, rather in the exquisite rhythm of movement than in the rigid collocation of the material world. We see only a few spokes of the great wheel of the universe; and what appears to be a destructive breach of continuity, may be only the completed part of a mighty cycle.

As to the purpose served by comets, the odd fancy of Newton, that they are intended to serve as fuel to the sun, is again revived, and revived by our ablest philosophers. It is held, in connexion with the dynamical theory of heat, that the sun's heat and light must be exhausted, unless supplied with fuel, and comets are held as part of the fuel destined to keep the central fire burning. It is supposed that there is a zone of aerolites, constituting the zodiacal light, which supplies the sun with the needful heat. And some careful observers state, that they have actually seen large bodies plunge into the luminous atmosphere, just as a large block of coal is thrown into the furnace by the stoker. These bodies are not supposed to act precisely as fuel, though they keep up the heat. In the practice of the Armstrong guns, it was found when a bolt was lodged in the sides of the gun-boat, that it could not be touched with the hand on account of the great heat—the arrested motion being converted into heat. So the aerolites and comets are supposed to communicate the requisite heat by having their prodigious velocity arrested, and converted into this other form of force. This is, however, a point of so much interest, and sheds so much light on the wise balancing of forces throughout the material world, that we reserve it for a separate paper.