presages of war. In all probability they belong to the class called by the ancients the Horn, one of the kinds of comets mentioned by Pliny. Examples of it are not unfrequently met with in ancient drawings; but we must not forget that the observers of former times were not always the most exact of draughtsmen, and that they did not hesitate on occasion to improve upon nature according to the dictates of their fancy. A curious instance of this mania for embellishment occurs even in the work of Hevelius. This indefatigable and learned philosopher, wishing to represent in his Cometographia the kind of comet which Pliny, under the name of Xiphias, has compared to a sword, has not failed to add the handle of the weapon. fac simile of this remarkable design has been given in Plate III., fig. 9.

A

Cometary tails are generally curved in the same direction throughout their whole extent; so that one of the boundarylines of the tail turning its concavity to one region of the heavens, the other boundary will turn its convexity to the region opposite; as, for example, the comet of 1811, Donati's comet, and many others. The two tails of the comet of 1807 were curved in opposite directions; and a drawing of the same comet of 1811, which we find in Chambers's Astronomy and in the Atlas of A. Keith Johnston, represents a similar phenomenon. A more exceptional form, and one of which we know no other example, is mentioned by Pingré in these terms: The late M. de la Nux, at the Isle of Bourbon, and ourselves, between Teneriffe and Cadiz, both remarked that the tail of the comet of 1769 was doubly curved towards its extremity; it resembled the figure of an.' But we should bear in mind that Messier has given several drawings of the same comet in which the tail is represented as a rectilinear band, brighter at its edges than either at its axis or in its interior. This last peculiarity is not un

frequent. Nevertheless, the contrary may occur, as was observed in the case of the comet of 1618. 'At Rome,' says

Pingré, there was seen a kind of nucleus, so called by Hevelius, in the tail of the last comet of 1618; it resembled a line or a dart, which, like the pith of a tree, extended the whole length of the tail, dividing its breadth into two parts. Kepler and Schickard saw the same phenomenon, but it did not then divide the breadth of the tail, it skirted along one of its edges, which is more in conformity with what is generally observed.'

Beyond the forms which we have just described, and which are sufficiently regular to admit of exact definitions, the tails of comets may assume irregular and whimsical appearances. In the accounts extant of great and historic comets, seen with the naked eye by observers who were often themselves astronomers, we find mention made of the most singular appearances; but we can hardly put faith in their descriptions, ingenuous perhaps, but certainly distorted by the superstitious beliefs of the times. It remains for modern astronomers to follow and to depict with scrupulous fidelity all the forms of cometary nuclei, atmospheres, and tails, as exhibited in the field of the telescope. The evolutions of these phenomena are but little known, and they must be studied without preconceived ideas, if we would fabricate a theory which should be exempt from the fallacies of observers. The sole means of discovering truth, in astronomy, as in all the natural sciences, is to begin by collecting facts, and then, relying upon them alone, to deduce reasons.

SECTION VII.

LENGTH OF TAILS.

Apparent and real dimensions of the largest tails on record-Formation and development of cometary appendages; their disappearance-Variations of length in the tail of Halley's comet at its different apparitions-Great comet of 1858, or comet of Donati.

SINCE we have entered upon the statistics of various cometary elements, let us here give a few particulars respecting the real and apparent dimensions of cometary tails. We will first confine ourselves to the maximum dimensions under which they have been viewed from the earth, dimensions measured in degrees, according to the apparent extent occupied by the train itself in the celestial vault. Passing, then, from the apparent lengths, we will proceed to the actual measures expressed in miles. Under the first head the scale of magnitude will be found to include an enormous range, varying from the tail of 20, belonging to the comet of 1851, to the immense tail of 100°, possessed by the comet of 1264, and to the still greater tail of the comet of 1861, which attained a length of 118°, thus exceeding by 28° the apparent distance between the horizon and the zenith. Nor are the differences less considerable when we compare the true dimensions. Whilst, for instance, the second comet of 1811 was provided with a tail about seven millions of miles in length, the great comets of 1811, I., 1847, I., 1687, and 1843 launched into