354 



KNOWLEDGE ♦ 



[O. 



THE MENACING COMET. 

 By R a. Proctor. 



,<-d /rom p«.jf 342.) 



singular discrepancy, wliick hi 



(Con I 



DR. HIND fouiul 

 been uuch misunderstood, between the movements 

 of the gr»-at comet and the result of his calculations. On 

 Sept 17, Mr. Gill's assistants at the Cape Observatory saw 

 the comet just touch the sun's limb, or situated as at A, M A.< 

 l>eing their line of sight From observations made on 

 Sept 1 ^ and 2"J, and on Oct '2, when the comet was 

 situated as shown at B, C, and D, Dr. Hind calculated an 

 orbit, which satisfied these observation.-; perfectly ; l>"/ 

 when the place of the comet was calculated backwards for 

 the hour when Mr. Oills a.ssistants saw it at A, it was 

 found that the comet should liave Ijeen as at a, the 

 line of sight to it from the earth projecting it in direc- 

 tion mat', or as seen at .•>', well on the sun's disc. Here 

 Dr. Hind rightly finds evidence of great disturbance^not 

 sayint; of what kind. Soin.' iiicxpfricnci'd per.sons have 



said that, so far from }>eing retarded, the comet was hastened 

 near perihelion, having traversed the distance API3 in 

 the time in which, according to the calculated orbit, it 

 Ahould have traversed the distance nPB only. The reverse, 

 however, is the case. ; Dr. Hind's calculations show that 

 the comet, according to the deduced orbit, would have 

 travelled the distance </PB only, or a less distance, in the 

 time in which it actually traversed the distance APB ; or, 

 in other words, the orliit deduced from its motion after 

 p«rih» lion pa.Hsage corresponded to a les-s vel>city than that 

 with which the comet had actually pas-sed the perihelion 

 part of itii orbit Hence the velocity after that passage 

 waJt a reduced velocity : and the comet must, therefore, 

 ntiirn in l«-«s time than its last 'ircuit occupied. 



It in not<-worthy, that a similar obvrvation was made 

 in tin- ca«e of the comet of l^'l.'J, though its significance 

 wan overlooked, astronomers not being then apparently 

 prcparf.-d to accept the idea of cometic retardation by the 

 resistance of the solar surroundings : — 



Professor Uenj. Peirce, the eminent American mathema- 



tician, announced that Captain Ray (described to liim, in 

 a letter from Mr. Mitchell, of Nantucket, as " a man of 

 sound judgment, a very accurate observer, and correct 

 man ') saw the comet of 181.'?, at 11 a.m., on February '27, 

 at Concepcion, S.A., liearing almost precisely ejist, with 

 scarcely any perceptible nortliing from the sun. The 

 comet was only live minutes of arc from the sun, or about 

 one-sixth of the sun's apparent diameter. This would 

 correspond with the position E on thi* orbit, and the line 

 of sight, liEn. Now, observations made later, as when the 

 comet was at F, O, H, were w<>ll represented by an orbit 

 which placed the comet on Feb. '27, 1 1 a.m., at Concepcion, 

 at a point such as *•, such that the line of sight, lev' passed 

 II.'') minutes of arc from the sun. From this it would 

 appear that even in passing from E to F, the comet had 

 been retarded ; for its motion, after passing F, corresponded 

 to such a rate as would have carried it over e F in a certain 

 portion of time, in which it actually traversed the larger 

 arc EF. 



I may add that Mr. Erck, a well-known observer, has 

 written a letter to the Irish Worlcf, in whicli he says, on 

 the strength of a single observation, that the comet is 

 moving in a parabola. l\c evidently misunderstands the 

 iiicthods of computation used in these cases. The assump- 

 tion of a parabolic suite suffices for ordinary observation, 

 for parts of the path near the sun, where the eccentricity is 

 very great even in the case of comets having known 

 c-lliptic orl)its. Mr. Erck found the comet on the predicted 

 track — and so he was certain to do. His inference is quite 

 erroneous that the comet is therefore travelling in a para- 

 liolic orbit 



NOTES ON THE COMET. 



We receive from France, Germany, Italy, and Greece 

 accounts of the Finlay Comet (as it should bo called, having 

 li(!en first seen by him). 



Ricco, who observed it at Palermo, notes that this ia the 

 first case of a comet which was shown to have passed peri- 

 JK^lion by the changes which took place in its spectrum. 

 On Sept -7 he found the spectrum of the nucleus con- 

 tinuous, but traversed by a strong line, that of sodium. 

 Knlarging the slit of the spectroscope, he saw a globular 

 image of nucleus and coma, formed by the orange-yellow 

 light of sodium. Many other lines besides the sodium line 

 were present ; but he could not determine their position. 

 I'hcre was a band in the red, a lino in the yellow near I), 

 two others in the green, and an enlargement of the con- 

 tinuous spectrum of the nucleus in the green and blue. 



Later, until October 1, the nucleus became less luminous 

 and appeared doul)le, and lengthened to 2", with a very 

 brilliant jet directed towards the sun. The tail was more 

 curved, and diverging, in length nearly Ifp, and at the 

 broader end 1° 48' in width ; the southern end much 

 stronger and brighter than the northern. A dark streak 

 seemed to divide the comet along its whole length. At this 

 time the comet was no longer so yellow as it had been ; 

 but the sodium line, though fainter, could still be seen. 

 The three bands seen in the spectra of so many comets, 

 and belonging to hydrocarbons, were now very conspicuous. 



After October 1 the tail was longer (about 17") and 

 about 2' 4H' wide at its widest part The three hydro- 

 carbon bands could be traced 5' from the nucleus. The 

 spectrum of the - tail continuous, and visible to the 

 extremity. 



Prof. Kriigor, at Kiel, noticed that on October 5, during 

 occasional moments of greater distinctness, the comet's 

 head appeared to have two distinct nuclei. 'J'his was con- 

 firmed on the 7 til. 



