November 1, 1887.] 



♦ KNO^A^LKDGE ♦ 



13 



Moreover there was this remarkable, and, indeed, 

 absolutely unique peculiarity about the orbit thus assigned : 

 the comet (whose period of revolution was to be measured 

 by hundreds of years) actually passed through the whole of 

 that part of its course during which it was north of our 

 earth's orbit plane in less than two hours and a half! though 

 this part of its course is a half-circuit around the sun, so 

 far as direction (not distance of travel) is concerned. That 

 comet, when at its nearest to the .sun, was travelling at the 

 rate of about 330 miles per second. It passed thi-ough 

 regions near the sun's surface commonly supposed to be 

 occupied liy atmosp'ieric tuattev. 



calculate the return of comets.) The comet of 1680, 

 called Newton's, was the very first whose orbital motions 

 were dealt with on the principles of Newtonian astronomy, 

 and Halley's was the first whose periodic character was 

 recognised. 



In 1843 another comet came up from the south, and 

 presently returned thither. It was, indeed, only seen 

 during its return, having, like the comet of l(i()8, bsen only 

 discovered a day or two after perihelion passage. Astro- 

 nomers soon began to notice a curious resemblance between 

 the orbits of the two comets. Remembering the compara- 

 tive roughness of the observations made in 1668, it may be 



The CoDstellatiou?, thougU'uunamed, can readily be identified, when it is noted that the 

 Comet's course, as here presented, began in the constellation of the Crane. 



Now, had the comet been so far checked in its swift rush 

 through tho.se regions as to lose one-thonsandth part of its 

 velocity, it would have returned in less than a year. But 

 the way in which the comet retreated showed that nothing 

 of this sort was to be expected. I am not aware, indeed, 

 that any anticipations were ever suggested in regard to the 

 return of the comet of 1668 to our neighbourhood. It was 

 not till the time of Halley's comet, 16S2, that modern 

 astronomy began to consider the question of the possiljly 

 periodic character of cometic motions with attention. (For 

 my own part, I reject as altogether improbable the state- 

 ment of Seneca that the ancient Chaldean astronomers could 



said that the two comets moved in the same orbit, so far as 

 could be judged from observation. The comet of 1843 came 

 along a path inclined at apparently the same angle to the 

 earth's orbit-plane, crossed that plane ascendingly at appre- 

 ciably the same point, swept round in about two hours and 

 a half that part of its angular circuit which lay north of the 

 earth's orbit-plane, and crossing that plane descendingly at 

 the same point as the comet of 1668, passed along appre- 

 ciably the same course towards the southern stellar regions ! 

 The close resemblance of two paths, each so strikingly 

 remarkable in itself, could not well be regarded as a mere 

 accidental coincidence. 



