112 SCENERY OF THE HEAVENS. 



miles. The tails appear to stream from that part of the nucleus which is farthest from 

 the sun, but seldom in the direction of a straight line joining the two bodies. They 

 generally exhibit a sensible curvature, bending towards that region of the heavens 

 last quitted by the comet, and cases have been observed in which they have formed 

 a right angle with the nucleus. The figure represents their common form, the arrow 

 showing the direction of the comet's motion, -and the dotted line 

 the direction of the sun. There is great enlargement in the 

 breadth of the tail, as its distance from the nucleus increases ; 

 and an obscure stripe appears passing down the middle, which 

 has suggested the hypothesis of a hollow luminous cone. It is 

 obvious that whether comets shine by inherent light, or reflect 

 the solar rays, if the train be a hollow cone, a much greater 

 number of nebulous particles will be- in the direction of the eye 

 at the sides than at the centre, which will account for the 

 interior dim stripe, and the exterior brightness. Comets, 

 however, are by no means confined to one train each, but as many as six have been 

 observed appended to the same nucleus. While these variations of form may be due 

 in a measure to different velocities, it is clearly ascertained that the aspect of the same 

 cometary body undergoes great changes in its period of revolution. In recognising 

 two apparitions as appearances of the same body, after having accomplished its periodic 

 time, astronomers do not depend upon the circumstances of shape, size, or brilliancy being 

 similar, but upon the elements of the path being accordant. 



Towards the close of the year 1680, a comet, illustrious on account of its observers, 

 and apparently formidable from its aspect, appeared within the visible limits of our 

 system, and approximated nearer to its centre than any body of which we have any 

 record. It finally vanished from terrestrial gaze in the month of March 1681, and has 

 not since been seen. The mind of Europe was profoundly impressed with the vast size, 

 velocity, and form of this object, which engaged the accurate observation of Flamstead and 

 Cassini, and the mathematical science of Bernouilli, Newton, and Halley. After its 

 perihelion passage, its appearance, as seen from Paris and particularly from Constantinople, 

 was most imposing. The train reached to the zenith when the nucleus had set below the 

 horizon, coruscations attending the whole length of the luminosity, giving to the phe- 

 nomenon the aspect of a wrathful messenger, and not that of a tranquil body pursuing a 

 harmless course. The greatest length of the tail was computed to be 123 millions of 

 miles, and in two days an extent of 60 millions of miles was emitted from the nucleus. 

 Its average velocity was upwards of 800 thousand miles an hour. A traveller through 

 our heavens, covering such a space, and rushing with such speed through the firmament, 

 might well excite the astonishment of mankind. It must not be imagined that this rate 

 of motion is its average orbital velocity. In obedience to the Keplerian law its pace 

 slackens in receding from the sun. According to the computation of Newton, this body 

 approached the sun within the 163rd part of the semidiameter of the earth's orbit, being 

 rather more than half a million of miles from his centre, and not more than 144,000 

 apart from his surface. If the projectile force had been stopped, in three minutes it 

 would have closed with his mass. In such a situation it must have been exposed to a 

 temperature which in an instant would dissipate any substance with which we are ac- 

 quainted. Newton calculated the body of the comet to have been heated to a degree two 

 thousand times greater than that of red-hot iron. This comet is supposed to be identical 

 with the one that appeared about the time of Caesar's death, with that which was seen in 

 the reign of Justinian in the year 531, and with another in the year 1106 in the reign 

 of Henry II. Comparing those dates, we find, from before Christ 44 to 531 leaves a 



