Four other comets have presented analogous phenomena, but with differences that we shall proceed to mention. These are the comet of Donati 1858, those of 1860, III., 1861, II., and lastly the comet of 1862, II., concerning which we shall enter into some details. These details will explain the formation and succession of the luminous aigrettes, or sectors, the nebulous envelopes to which they give rise, and lastly the formation of the tail, which the cometary matter that has

Fig. 40.-Formation of luminous sectors and Fig. 41.-Comet of 1860, III. June 27, acenvelopes. Donati's comet, Sept. 8, 1858. cording to Bond. Aigrettes and envelopes.

thus left the nucleus appears to originate under the influence of a kind of repulsion, the cause of which we shall have later on to consider.

In Donati's comet the jets of luminous matter liberated from the nucleus in the form of luminous sectors, disposed like a fan, produced around the head successive envelopes, which as they receded from the nucleus diminished in brightness and became uniformly blended. This kind of compression was

regarded by Bond as the result of progressive diminution in the velocity of expansion of each envelope. Seven successive

envelopes, rising above the nucleus, were formed in periods varying from 4 days 16 hours to 7 days 8 hours. Each successive envelope as it arose remained as if retained by the nucleus for a certain time, until, in virtue of some acquired property, it drifted back and contributed to form the two main divisions of the tail. The sectors always appeared in the same direction,

Fig. 42.-Luminous envelopes of Donati's Fig. 43.-The same comet. October 2. comet. September 30, 1858. From a drawing by Bond.

facing the sun, so that we may conclude that neither the nucleus nor the coma was endowed with a movement of sensible rotation; thus no oscillation of the kind observed by Bessel was manifested in the head of Donati's comet, except the motion necessitated by the constant direction of the luminous sectors towards the sun. Even this absence of rotation, according to Bond, implies the action of a polar force, ema

nating from the sun, and maintaining the axis of the nucleus in the direction of the focus of movement.

The comets of 1860 and 1861 were also the seats of nucleal emissions in a permanent direction, the first for a fortnight, the second for a month. Eleven successive envelopes

Fig. 44.-Formation of the luminous envelopes Fig. 45.-The same. October 8. Both in Donati's comet. October 6. from drawings by Bond.

were emitted from the nucleus of the comet of 1861, at regular intervals of two days. The development and final dispersion were thus accomplished with much greater rapidity than in the case of Donati's comet.

SECTION II.

OSCILLATIONS OF LUMINOUS SECTORS: COMET OF 1862.

M. Chacornac's observations upon the comet of 1862-Formation of luminous sectors emanating from the nucleus-Oscillation of aigrettes, and flowing back of the nucleal matter.

WE are now about to give our attention to the evolutions of the luminous sectors of the great comet of 1862, which, on the contrary, presented oscillations analogous to those exhibited by the aigrettes of Halley's comet. We shall follow the development of these phenomena by means of the observations. of the late M. Chacornac.

On August 10, 1862, M. Chacornac detected in the head of the comet the presence of a luminous aigrette, a brilliant sector directed towards the sun. This sector, which at three o'clock in the morning included an angle of 46°, had, by two o'clock on the following day, opened like the corolla of a convolvulus, and included 65°. On the 10th the nucleus presented the appearance of a rocket, having a diameter much more extended in the direction of the radius vector than at right angles to it.' It is worthy of remark that the contrary was the case with the nuclei of the comets of 1858 and 1861, which were flattened in the direction of the radius vector. On the 11th the two diameters were nearly equal. New sectors disengaged themselves successively from the nucleus, and on August 26 M. Chacornac determined that between the 10th

and the 26th they had succeeded each other to the number of thirteen.

Having carefully followed throughout this interval (with the exception of three nights, when the sky was cloudy) the formation of these successive sectors, M. Chacornac has given in the following terms a brief description of the phenomena which he observed:

'The nucleus of the comet emits periodically, in the direction of the sun, a gaseous jet from which particles of cometary matter escape like steam escaping from a piston. This jet preserves for a certain time a rectilinear form, as if a force of considerable projecting power, residing in the nucleus, threw off particles in that direction; then it becomes inflected and takes the form of a slightly arched cone. At this same moment the cometary matter, accumulating at the extremity of the jet nearer to the sun, forms a kind of cloud, the rounded outline of which would appear to indicate that at this distance from the nucleus the force of projection has been overcome by some resistance opposed to it; the cloud returns on both sides, like a puff of smoke driven back by the wind; and, opening out into a level sheet, flows away in the direction of the tail.'

'By degrees the vaporous cone, the axis and vertex of which have continued to appear the most luminous portions, assume a diffused and nebulous appearance, as if veiled by an accession of thick atmosphere; the brightness of the centre diminishes, that of the sides increases, and the cone enlarges. The diffused appearance continuing to increase, the gaseous jet loses its form, the light of the axis disappears, and everything seems to indicate that the nucleal emission has ceased in this direction. The nucleus appears round and brilliant. At this time, at an angle with the radius vector of about 30 degrees towards the east, appear the first traces of a new jet destined to succeed the former; and, in proportion as these