thirty days, by any disturbing cause, then, indeed, the earth and comet would have filled at the same time the point where their orbits intersect, and the dreaded collision would have taken place. The consequences of such a shock it is impossible to conjecture, but reasoning from the known physical condition of the comet, none of the terrible disasters so generally anticipated would have occurred. The exceeding rarity of the matter composing this body may be inferred from the statements of Sir John Herschel. “It passed," says he, "over a small cluster of most minute stars of the 16th and 17th magnitude; and when on the cluster presented the appearance of a nebula resolvable, and partly resolved, the stars of the cluster being visible through the comet. A more striking proof could not have been offered of the extreme translucency of the matter of which the comet consists. The most trifling fog would have effaced this group of stars, yet they continued visible through a thickness of cometic matter which, calculating on its distance and apparent diameter, must have exceeded 50,000 miles, at least towards its central parts. That any star of the cluster was centrally covered, is indeed more than I can assert; but the general bulk of the comet might be said to have passed centrally over the group."

Such is the nature of the body from whose contact the ignorant apprehended the most fearful convulsions. Olbers, who studied the subject with great care, was disposed to think that in case the earth had passed directly through the comet, no inconvenience would have occurred, and no change beyond a slight influence on the climate would have been experienced.

It is useless to speculate with reference to the probable consequences of a collision, which there is scarcely one chance in millions can ever occur.Science has as yet discovered no guarantee for any planet against the possible shock of a comet; but an examination of the delicate adjustments of our own system, and those of Jupiter and Saturn, would seem to indicate to us that in all past time no derangement has ever occurred from such a cause.

The last return of Biela's comet was marked by a phenomenon unexampled, so far as I know, in the history of these wandering bodies. True to the predictions of Santini, the comet first became visible on the evening of the 26th of November, 1845, and in the precise point which had been assigned by theory. DeVico, the director of the observatory at Rome, was the first to catch a glimpse of the expected comet.— Nothing remarkable in its appearance was noticed, until about the 29th of December, wen Mr. E. C. Herrick, of New Haven, pointed out to several friends what he regarded to be a sort of anomalous tail, but shooting out from the head of the comet in a direction. entirely at variance with the usually received theory, that the tail is always opposite to the sun. In this suppose:l tail a kind of knot was noticed, brighter and more condensed than any other part. Owing to insufficient optical power, the true character of the phenomenon was not fairly detected by Mr. Herrick.

On the night of the 12th of January, 1848, Lieut. Maury, in charge of the observatory at Washington City, discovered that what had hitherto appeared as a single body, was actually composed of two distinct and reporute comets

This most extraordinary fact was

immediate.y announced, and the double character was observed at all the principal observatories in Europe and the United States. There can be no doubt whatever as to the reality of the appearance. The comet actually became double, and the two parts bound together by some inscrutable bond, continued their swift journey through space, pursuing almost exactly the route predicted for the single comet.

From measures obtained by Prof. Challis, of Cambridge, England, on the 23d of January, 1846, the two comets were separated from each other by a distance equal to about one-thirteenth the apparent diameter of the sun. On the 28th of the same month, Sir John Herschel records the following notices :"The comet was evidently double, consisting of two distinct nebulæ, a larger and a smaller one, both round, or nearly so,-the one in advance faint and small, and not much brighter in the middle; the one which followed nearly three times as bright, and one and a half times larger in diameter, and a good deal brighter in the middle, with an approach to a stellar point."

On the evening of the 9th of February, having returned to the observatory at Cincinnati, after an absence of more than two months, I had an opportunity of beholding, for the first time, these wonderful objects, with the twelve inch refractor. The moon was nearly full, and yet the comets were distinctly visible, both included within the limits of the field of view of the instrument, and separated from each other by a distance equal to about the eighth part of the sun's diameter. The preceding comet was evidently the brighter of the two

Clouds prevented a continuous examination of the comets from night to night, but on the evening of the 21st of February, I was surprised to find a remarkable change in the relative brilliancy of the two parts. On that evening the following comet was very decidedly brighter than its companion, and from observations made elsewhere, the change of relative brightness seems to have been effected about the 13th or 11th of February. The change was observed by Prof. Encke, of Berlin, as early as the 14th. On the evening of the 21st of February, both comets exhibited distinct trains of light, extending from the sun, and in directions parallel to each other. The centre of the nucleus of each comet was brighter than the surrounding portions, but there was no stellar point visible. The nebulosity of the two points did not intermingle.

The distance between the comets increased from day to day, until, on the 25th of February, they were separated by an amount equal to 445 seconds of arc, or between a fourth and fifth part of the sun's diameter. A part of the increase of distance was only apparent, arising from the approach of the comets to the earth, but the comets were actually receding from each other while pursuing their rapid flight through space.

Neither did the line joining the central points of the comets remain parallel to itsel. From the 23d of January to the 11th of February, this line shifted its position by an amount of angular motion equal to 8° as is shown by a comparison of the measures of Challis and Encke. By the 21st of February, this angular motion had been nearly destroyed by a retro

grade movement, and thus the comets were seen to oscillate about each other, according to some mysteri ous law which has never been revealed. Such is a brief sketch of the phenomena presented by Biela's comet in its late return. Its next appearance will be looked for with deep interest, to confirm or destroy certain theories which have been propounded to explain its duplex character.

While the periods of the comets which we have thus far considered are comparatively short, those of others which have visited our system have been ascertained to extend to many thousands of years. The great comet of 1811, one of the most brilliant of modern times, in consequence of its remaining visible for nearly ten months, gave ample opportunity for the investigation of the elements of its orbit. After a careful investigation, M. Argelander fixes its period of revolution at 2,888 years. Bessel had examined the same subject previously, and probably with less attention, but obtained a period even greater than Argelander's, amounting to 3,383 years.

The comet of 1807 also occupied the attention of Bessel. A long series of observations furnished the data for computing its elements. The periodic time was fixed at 1,543 years. These computations are necessarily only approximate. The difficulty of ob taining accurately the periodic time increases with the length of the period, and all that can be done is to fix a limit below which it cannot fall. These vast periods give to us the means of learning somewhat of the great distance to which these objects penetrate into space. The comet of 1811, having a period probably three thousand times greater than that of