THE TWO COMETS OF THE YEAR 1868. 161 



neither of these causes could suffice to account for the 

 great extension of these appendages. Then the rapid 

 manner in which the tails seem to be formed remains 

 wholly mysterious. And we are also left without any 

 explanation of the rapid change of position exhibited by 

 the tail while the comet is sweeping around the sun 

 at the time of nearest approach to that luminary. 

 Sir John Herschel compared this motion to that of a 

 stick whirled round by the handle the whole extent 

 of the tail partaking in the movement as if comet and 

 tail formed a rigid mass. 



The difficulties here discussed seem in the present 

 state of our knowledge wholly insoluble. In fact, it 

 seems impossible even to conceive of a solution to the 

 last mentioned phenomenon, so long as we look upon 

 the comet's tail as a distinct unvarying entity. For 

 instance, if the tail, a hundred millions of miles long, 

 which extended backwards from Halley's comet before 

 perihelion passage, consisted of the same matter as the 

 tail which projected forwards to the same extent a few 

 days later, then certainly there is nothing in our pre- 

 sent experience of matter and its relations which can 

 enable us to deal with so astounding a phenomenon. 

 It will be understood, of course, Sir John Herschel does 

 not say in so many words, that the tail of Halley's 

 comet was brandished round in the manner described 

 above, but that, although it appeared to move in 

 this manner, it is impossible so to conceive of its 

 motion. 



M 



