June, 1913. 



KNOWLEDGE. 



235 



from circles in most cases, the comets in general are 

 found to move in orbits so nearly parabolic (whilst 

 in the small part of their path during which they are 

 visible to us), that only a few are known whose 

 motion is otherwise, all three curves differing but 

 little near the vertex. Thus the orbit of a newly- 

 discovered comet is always first calculated on the 

 assumption that its motion is parabolic ; another 

 reason being that the necessary calculations are 

 simpler than for an elliptic or hyperbolic orbit. A 

 strictly parabolic orbit, however, is practically impos- 

 sible, for the smallest diminution of speed due to 

 planetary action would change it into an ellipse, 



The vast majority appear to move approximately 

 in parabolas, a considerable (and increasing) number 

 in ellipses whose deviation from circularity is much 

 greater than that of the planetary orbits, but not 

 always so great as that of some double-star orbits. 



In 1682, a comet was observed by Newton, Halley 

 and others, and on examining the circumstances of 

 its motion Edmund Halley computed its orbit on the 

 supposition that the latter was a parabola. Com- 

 paring his results with former observations and 

 computing other orbits from the necessarily 

 imperfect determinations of position of earlier days, 

 he found that in 1531 and 1607, comets had 



Figure 242. Comet 1908 III (Morehouse). 



E. E. Barnard, Yerkes Observatory. 1908, Oct. 15 d 12* 57 m G.M.T. Position of Comet: R.A. 19 h 27'" ; 

 Decl. + 50° 16'. Bruce Doublet Lens: Ap. 10-in. (0-254m) ; F.L. 50-3-in. (l-28m). Exp. 82 min. 



whilst an increase would convert it into a hyperbola. 

 The remarkable fact that so many cometary orbits 

 closely approximate to a parabolic form must have 

 important bearings on any theory as to their origin, 

 though there are reasons for suspecting that the true 

 orbits are not really parabolic, but very long and 

 eccentric ellipses. A comet moving in a parabola (or 

 hyperbola) will be seen only once, never to return ; 

 whereas, one moving in an ellipse, the latter being a 

 closed path, must return again sooner or later, 

 though thousands, or even perhaps millions, of years 

 may be required to complete one revolution round 

 the sun, their centre of force ; the common focus of 

 their orbits. A very few comets have been suspected 

 to move in hyperbolic orbits, but the deviation in 

 excess of parabolic velocitv is always so small, that 

 it may reasonably be doubted whether any known 

 hyperbolic-moving comet has been certainly detected. 



appeared which moved so nearly in the same path as 

 this one that he ventured to assert its identity with 

 them, and to predict its return in 1758 or 1759. 

 Though he could not expect to witness this event 

 himself he wrote on the subject : " If it should 

 return according to our prediction about the year 

 1758, impartial posterity will not refuse to acknow- 

 ledge that this was first discoveredby an Englishman." 

 It is well known that his prediction was completely 

 verified, and the comet's latest return (after 1758 

 and 1835) is so recent that the details must be 

 familiar to all readers of " Knowledge." The 

 true glory of a nation, not obtained at the expense 

 of any other, nor involving the sacrifice of precious 

 lives, is shown more by such achievements than by 

 deeds of homicidal conquest, and we may be justly 

 proud of the names of our countrymen Newton, 

 Halley, Herschel, Cowell, Crommelin, and so on, 



