236 



KNOWLEDGE. 



June, 1913. 



in connection with the history of this comet, whilst 

 not detracting from the honour due to Clairaut, 

 Pontecoulant, Max Wolf and other " foreigners " 

 who have assisted in the same field. At its last 

 return the brilliant development of the tail 

 phenomena, and so on, made the comet an object of 

 great beauty and interest to observers in more 

 favoured latitudes than our own (where the prevalence 

 of moonlight and the short summer nights prevented 

 or hindered observation) showing that the idea that 

 the comet, through the loss of matter driven off at 

 former returns, would be less conspicuous than of old, 

 was groundless. In fact, it is safe to assert that at 

 no previous return known to history has the comet 

 been so brilliant, whilst its early discovery by photo- 

 graphy and long visi- 

 bility have enabled its 

 changes from day to day 

 to be registered with a 

 completeness hitherto 

 unapproached. In addi- 

 tion to the physical 

 phenomena of interest 

 developed, it is of in- 

 terest to note that there 

 remains an outstanding 

 difference between 

 theory and observation, 

 whereby the comet's re- 

 turn to perihelion occur- 

 red three days later than 

 the date indicated by 

 the final previous cal- 

 culations of Drs. Cowell 

 and Crommelin, which 

 difference is not due to 

 the action of any known 

 material in the Solar sys- 

 tem. This may, per- 

 haps, be accounted for 

 as the result of the action 

 of an unknown planet, 

 but the present writer 

 has suggested that this 



may be an effect of the resisting medium. Halley's 

 comet moves in the retrograde direction (that is to 

 say "clockwise" or opposite to that in which the 

 planets and many of the known periodic comets 

 move), and as there is some evidence that 

 Encke's comet (a direct moving comet to be referred 

 to directly), is slightly accelerated by such an action 

 it may not be unreasonable to suppose that the 

 opposite effect may be experienced by a retrograde 

 one. The comets are bodies of such infinitesimal 

 mass, combined with great volume, that their density 

 must be inappreciably small, so that an action quite 

 unobservable for the more massive planets may well 

 be sensible for them. The most remarkable of the 

 regularly returning comets is that of Encke. Its 

 periodicity was first detected by the astronomer of 

 that name, from the comparison of the orbits of 

 comets which appeared in 1786, 1795 and 1805, with 



that which was discovered by Pons in November, 

 1818, and from his researches on its motion, con- 

 tinued up to the time of his death, the comet has 

 ever since been known by the name of Encke's 

 comet. It has the shortest period of any known 

 comet, rather over one thousand two hundred days. 

 But it was not long after it had been observed at 

 some following returns (1825, 1829 and 1835) that 

 Encke found a slow progressive diminution of its 

 period, and was led to conjecture the existence of a 

 " thin ethereal medium," which, continually resisting 

 the comet's motion, drew it slightly nearer towards 

 the Sun than would otherwise be the case, this 

 diminution of distance (by Kepler's third law) 

 involving a diminution of period. This diminution 



continued at the follow- 

 ing returns, after the 

 death of Encke. His 

 comet has been carefully 

 studied by Von Asten, 

 and more recently by 

 Dr. Backlund, for whose 

 researches the gold 

 medal of the Royal 

 Astronomical Society 

 was given in 1909. He 

 finds that the diminution 

 in period has not been 

 quite regular, but has 

 undergone changes, fall- 

 ing, after 1858, to three- 

 fourths its former value 

 between 1861 and 

 two - thirds only 

 1871 to 1895, and 

 1895 having had 

 half the value it 



1868, 

 from 

 since 

 onlv 

 had 

 then, 



Figure 243. Comet 1908 III (Morehouse). 



P. J. Melotte and C. R. Davidson, Royal Observatory, 



Greenwich. 1908, Oct. 30 d 7 h 47 m G.M.T. Position of 



Comet: R.A. 18 h 59 m ; Decl. + 25° 18'. Reflector: Ap.30-in. 



(0-762m) ; F.L. 11-ft. 5-in. (3-48m). Exp. 10 min. 



before 1860. If, 

 we attribute this " ac- 

 celeration" to the action 

 of a resisting medium, 

 which tends to bring it 

 nearer the sun, we 

 obtain the somewhat 

 paradoxical result that the nearer the comet ap- 

 proaches the sun the less is its motion affected. 

 But the seeming contradiction may be explained if, 

 as the late M. Eaye suggested, the medium has some 

 motion of its own. " A comet moving in a resisting 

 medium of this kind will not be precipitated upon 

 the Sun, but the principal effect will be a change 

 (diminution) in the eccentricity of its orbit. This 

 will become more and more nearly circular, but after 

 a time the axis will no longer decrease." In fact, a 

 stationary resisting medium is incomprehensible, 

 unless it be infinite in extent, for otherwise it would 

 have been long since precipitated upon the surfaces 

 of the sun and planets. Professor See has shown 

 the action of a resisting medium in past ages in 

 effecting the approximate circularity of the planetary 

 orbits, and here we seem to have indications of the 

 present action also of such as still remains 



