June, 1913. 



KNOWLEDGE. 



237 



unabsorbed by the larger bodies of our system. As 

 our knowledge of cometary orbits becomes more 

 exact, it seems reasonable to suppose that other 

 comets will be seen to be similarly affected. 

 Faye's comet of 1843, Winnecke's comet (1858 ii) 

 with a period of six years, and one or two others 

 (Tempel Swift's and Brorsen's comets) have also 

 been suspected to show 

 some evidence in their 

 motion of such action, 

 but the evidence is 

 somewhat conflicting, 

 since the difficulties in 

 the way of determining 

 the "undisturbed orbits" 

 of comets with sufficient 

 accuracy to prove so 

 small a "residual effect " 

 are many. A comet 

 cannot have its place 

 fixed with the precision 

 that is possible for a 

 planet. The perturba- 

 tions due to planetary 

 action are very import- 

 ant, and few comets 

 have been observed at 

 ' more than five or six 

 returns over very 

 limited portions of their 

 orbits. Halley's and 

 Encke's comets alone 

 have been observed at 

 many returns, and it 

 happens just for these 

 that we do seem to 

 have some evidence of 

 the action of a re- 

 sisting medium, ac- 

 celerating the direct 

 moving comet and 

 retarding the retrograde 

 one. In addition to 

 Faye's explanation of 

 the varying effect of 

 resistance, Dr. Backlund 

 isinclined to think 

 that (neglecting possible 



electrical actions) there may be a resisting medium 

 forming a sort of meteoric ring, whose density 

 diminishes as we approach the Sun within certain 

 limits, and that the diminution of resisting effect 

 may be a measure of this. Sir John Herschel 

 suggested that since the phenomena of comets' tails 

 show that matter is violently repelled from the 

 comet by solar action, the loss of such matter at 

 each return will render its proportion to the attracted 

 material less, and thus the " effective mass" will 

 increase, and, consequently, the comet will complete 

 each revolution in a shorter time. But it seems 

 difficult to imagine how such a change can produce 

 an effect of several hours' diminution in the period of 



P- J- 



Greenwich 

 Comet 

 (0 



1908, 



Nov. 19 d 6 h 



Encke's comet, and, of course, it cannot explain the 

 retardation of Halley's comet. 



Amongst the most remarkable of recent comets 

 we may easily give the foremost place to that 

 discovered by Morehouse on September 1st, 

 1908, which was continuously and extensively 

 observed. (See Figures 239-245). It underwent 



many remarkable 

 changes during that 

 time, and considerable 

 discrepancies occurred 

 in the accounts of its 

 peculiarities. At its 

 first detection, and for 

 some weeks later, it 

 was a somewhat diffuse 

 cloud}' - looking object, 

 with barely a trace of 

 nucleus and a short 

 tail, which was even 

 visible to the naked 

 eye at Copenhagen on 

 September 20th. To- 

 wards the end of the 

 month, however, changes 

 took place resulting in 

 a complete disruption, 

 and on the 30th the 

 tail was entirely gone. 

 Photographs taken on 

 October 2nd show new 

 faint tails, whilst on 

 October 14th a tail at 

 least 7° long was shown. 

 The following day the 

 comet had broken in 

 two. Photographs 



taken in the United 

 States show two great 

 condensations in the 

 tail, about J° from the 

 head : a bright, short, 

 spike - like projection, 

 with one end between 

 the two masses and 

 the broad end attached 

 to the comet, formed 

 the new tail (Chambers). 

 Further changes took place from day to day. End- 

 less streamers shot out from the main body of the 

 tail, which was violently bent and twisted as though 

 it had encountered a resisting medium (Novem- 

 ber 15th). On November 19th, straight jets were 

 given off by the head. The comet repeatedly lost 

 its tail and formed new ones, which varied in type, 

 " condensations, waves, straight rays, and twisted 

 funnels made up its wonderfully active tail." 



The spectroscopic observations of this comet were 

 not less remarkable than the visual ones, and have 

 led to a great increase in our knowledge of cometary 

 physics. Admirable photographs were taken by the 

 Comte de la Baume Pluvinel and M. Baldet, with 



Figurk 244. Comet 1908, III (Morehouse). 

 Melotte and C. R. Davidson, Royal Observatory 



4 m G.M.T. Position of 

 R.A. 18" 51 m ; Decl. + 2° 20'. Reflector: Ap. 30-in. 

 762m) ; F.L. lift. 5-in. (3-48m). Exp. 30 min. 



