. i an. 



i MM ITS. 



71 



the following enumeration of the number of comets whoee orbiu bare 

 been calculated down to UM cloee of the year 1858 : 

 7 apparition* of Haller'i comet 

 14 apparitiona of Encke's comet. 

 6 apparitiona of Biela's comet. 

 9 apparitiona of Kaye'a comet. 



40 apparitiona of oomeU revolving in elliptic orbita, or of which two 

 inteagai of the perihelion may perha|ia have occurred. 

 151 apparitiona of comet* revolving in parabolic orbita. 



The following itatement of the progreaa of cometanr discovery during 

 the five yean which nave elapsed aubaequently to the year 1853, will 

 enable the reader to form an opinion with reapect to the activity which 

 pervade* thia department of agronomical science : 



Number of OomeU 

 Tear. discovered. 



185J 

 1856 



1M7 



1819 



It ia to be remarked that the foregoing list include* four re-discoveries 

 of periodic comet*, namely, the re-discoveries of Bronen's and D'Arrest's 

 cometa in 1857, and of Encko'a and Faye'a comeU in 1858. 



Dimauiont of Cornell. 



The head of a comet U frequently a body of immense magnitude, 

 The following are some of the results which have been obtained by 

 the aid of micrometxic measurements : 



Diameter of the Head. 



The great comet of 1811 

 Htllrv** comrt (1836) . 

 Pint comet of 1780 

 Comet of Lexcll, 1770 

 Donati'i comet 



. 1,170,000 mile* 

 . . . 857,000 

 267,600 

 . . 204,000 

 100,000 



Sir William Hcrschel found the diameter of the envelope of the great 

 oomet of 1811 to be no less than 643,000 miles. 



The nucleus of a comet, in many instances not distinguishable at all, 

 is generally of inconsiderable dimensions. The following are some 

 results of micrometric measurement : 



Diameter of Nucleus. 

 Comet of 1799 ....... 373 mile* 



1807 ....... 518 



1811 ....... 2637 



Donati'i comet (October 6, 1858) . . . . 800 



The tail of a comet sometimes attains an enormous length, aa the 

 following numbers will show : 



Length of Tail. 



Comet of 1680 ..... 96,000,000 mile* 

 Comet of 1769 ..... 38,000,000 

 Comet of 1811 (October 15) . .100,000,000 

 Comet of 1843 ..... 150,000,000 

 Great comet of 1858 (Donti') . . 91,000,000 



Physical Cunititution of Cornell. 



The subatance of which a comet is composed appears to be of remark- 

 able tenuity. Thia has been abundantly proved by the circumstance 

 of the smallest stars being seen through their structure without 

 undergoing any sensible diminution of light. Sir John Herschel, in a 

 paper on Biela's comet, published in vol. vi. of the ' Memoirs of the 

 Astronomical Society,' has mentioned a fact which affords a striking 

 illustration of the translucency of cometic matter. The comet having 

 paamd over a small cluster of stars of the sixteenth or seventeenth 

 magnitude, the appearance presented was that of a nebula, partly 

 resolvable into stars. The most trifling fog would have effaced the 

 atars ; but in the present instance they still continued to be visible, 

 although the cometic matter interposed between them and the observer 

 must have been at least fifty thousand miles in thickness. 



The queation whether the nucleus of a comet in in any case a solid 

 body ha* been often diacuaaed, but no definitive conclusion has been 

 arrived at. The paaaage of the nucleus of a comet over a star might 

 be supposed to supply a useful criterion for deciding this point, but no 

 instance of the actual occurrence of such a phenomenon has ever.been 

 aatisfactorily established. Newton was of opinion that the nuclei of 

 cometa mint nccemarily be solid bodies, since otherwise they would 

 in many caaes be diatipated in space by the intense heat to which they 

 are subjected on the paaaage of their perihelia. 



But whether the nuclei of cometa be solid or not, it U certain that 

 their maaae* must be very inconsiderable. This is evident from the 

 circumstance of their producing no aenaible derangement in the motion* 

 of the planet*, however near they approach them. In the year 1779 

 Lexell comet pasted through the middle of the system of Jupiter's 

 iteUite*, but none of those bodie* appeared to be in the slightest 

 degree affected by its attractive force. 



Allusion ha* been made to the great beat which many cometa mutt 

 undergo on their paaaage of the perihelion. The following table of 



perihelion distance* will ahow how near comet* in some instances 

 approach the sun : 



Date of Apparition Perihelion 



of the Comet. DUunce. 



|4> 475,000 mile* 



16(0 570,000 



1689 1,900.000 



1816 1,565,000 



1847 . 8,990,000 



1616 4,560,000 



Newton found by calculation, that the comet of 1880, on it* passage 

 of the perihelion waa subjected to a heat 2000 time* greater than that 

 of red-hot iron. The great comet of 1843, which approached nearer 

 the sun than any other comet recorded in history, must have been 

 exposed to a heat of still greater intensity. Laplace, availing himself 

 of Black's beautiful discovery of the principle of latent caloric, con- 

 sidered that the heat abstracted by the cometic particle* in the course 

 of passing into the vaporous state would serve to moderate the effect 

 of the solar heat at the perihelion, and upon this ground he concluded, 

 that the nucleus of a comet is not necessarily a solid body. 



When one of the more conspicuous comets is advancing towards the 

 perihelion, it is seen to undergo a succession of changes in the head 

 and tail. These singular phenomena, which are evidently due to the 

 action of the sun, appear to have been first remarked by Hooke in the 

 course of his observations of the comets of 1880 and 1682. The 

 following extract from his observation* of the comet of 1682 (an appari- 

 tion of Halley s comet), will enable the reader to form some idea of the 

 changes to which wo refer. It U right to state that the comet passed 

 through the perihelion on the 1 5th of September : 



" August 26. At seven in the evening I delineated the figure and 

 shape of the comet, exactly like that I saw through my fourteen-feet 

 telescope, which will appear more plain by the fifth figure than I can 

 otherwise well express it.* It had a pretty bright round nucleus, and 

 about that was an atmosphere of thinner light which was terminated 

 towards the sun with a round figure. That part of this halo, or lighter 

 atmosphere towards the sun, was not so bright or radiant as another 

 kind of light which seemed to issue from the nucleus or star 

 ways at right angles with the axis through the sun, which lighter 

 i sailings bent into a kind of parabolic figure, within the former halo or 

 atmosphere, and was terminated within it, and seemed to form, as it 

 were, a second parabolical termination towards the sun, in the apex of 

 which parabola was the bright nucleus, and this brighter parabolic line 

 of light seemed as gross or thick as the nucleus itself. This issued on 

 both Bides, but that on the right bond, or the northernmost, was much 

 more conspicuous ; insomuch that that on the left hand, or towards 

 the south, was to be seen but sometimes, but that on the other side 

 was very plain and conspicuous, and seemed like a stream of flame 

 blown out of a candle by a blowpipe, ascending or bending upwards, 

 just as such a blown flame of a candle will do, if it be made by a 

 gentle blast. This I remarked vegr carefully, to see whether I could 

 find, by any succeeding observations, any alteration of the magnitude, 

 figure, brightness, or position, in respect of the comet's axis. These 

 two bright spoutings of flame or light turned or bent upward* from 

 the sun, and after a short space seemed to unite into the axis or 

 middle of the blaze, and form the shape of the outside of a flame of a 

 candle tapering to a point : the fainter part also without it seemed 

 to taper much in the some manner. I saw also several coruscations 

 or flashings of the flame, shooting out to a great distance into the 

 blaze." 



Phenomena of a similar nature were remarked by Heinsius in the 

 course of his observations of the great comet of 1744. In more recent 

 times, Halley's comet (1835), and the great comet of 1858, exhibited 

 analogous changes previous to their passage of the perihelion. It has 

 been already stated that Halley's comet passed through the perihelion 

 on the 15th of November. Previous to the 2nd of October, the ap- 

 pearance which it presented was that of a round nebulous disk, with a 

 faint nucleus in the centre. On that evening, however, the nucleus 

 became exceedingly bright, and there was seen to issue from it a cone 

 of light, which first extended a short distance in the direction of the 

 sun, and then bent back as if impelled by some intense force in the 

 opposite direction. This outstreaming cone of light continued to be 

 seen until the 22nd of October, subject however to violent change* 

 when observed from night to night. It is worthy of remark that 

 simultaneously with these changes the tail was observed to increase 

 gradually in length. The phenomena observed during the apparition 

 of the great comet of 1858, were of a still more complex nature than 

 those which characterised any previous comet, but it would be out of 

 place here to enter into any minute details. On the 16th of September 

 there commenced a series of luminous emissions from the nucleus, 

 which continued till the passage of the perihelion at the end of the 

 month. These were followed by a succession of envelopes of a para- 

 boloidal form surrounding the head, and which were seen in greater or 

 leu number at the various observatories throughout Europe and 

 America till the disappearance of the comet about the 20th of October. 



The development of the tail when a comet is advancing towards the 

 perihelion affords also a striking indication of the action of the sun, 



TbeM drawing! ar* given In Hooke'i ' Foithamoui Works.' 



