comets' tails, the corona, and the aurora. 183 



was observed suddenly to h:ive become much brighter and to ])e in 

 the act of throwino- out a Jet or strc^am of light from its anterior ])art, 

 or that turned towartl the sun. This ejection, after ceasing a while, was 

 resumed, and with nmch greater apparent violence, on the 8th, and 

 continued with occasional intermittences so long as the tail itself con- 

 tiiuied ^•isible. '^ "'^' ■• These jets, though very bright at their 

 l)oint of emantition from the nucleus, faded rapidly away, and l)ecame 

 ditiused as they expanded into the coma, at the same time cur\ing 

 backward as streams of steam or smoke would do if thrown out from 

 narrow oritices more or less obliquely in opposition to a powerful 

 wind, against which they were unable to make way, and ultimately 

 yielding to its force, so as to be drifted back and confounded in a 

 \aporous train, following the general direction of the current. 



"It seems impossible to avoid the following conclusions: First. 

 That the matter of the nucleus of a comet is powerfully excited and 

 dilated into a \aporous state l)v the action of the sun's rays, escaping 

 in streams and jets at those points of the surface which oppose the 

 least resistance. Second. That this process chiefly takes place in that 

 portion of the luicleus which is turned toward the sun, the vapor 

 escaping chielly in thtit dii'ection. Third. That when so emitted it 

 is prevented from proceeding in the direction originally impressed on 

 it by some force directed from the sun drifting it back and carrying 

 it out to vast distances behind the nucleus, forming the tail. Fourth. 

 That this force, whatever its nature, acts unequally on the materials 

 of the comet. Fifth. That the force thus acting on the materials of 

 the tail can not "possibly ])e identical with the ordinary gravitation of 

 matter, being centrifugal or repulsive, as respects the sun, and of an 

 energy very far exceeding the gravitating force toward that lumi- 

 nary. This will l)e evident if we consider the enormous velocity with 

 which the matter of the tail is carried backward, in opposition l)oth 

 to the motion which it had as part of the nucleus and to that which it 

 acquired in the act of emission.'" 



Again, descrit)ing the long, straight tail of the great comet of 1843, 

 from which a lateral tail, nearly twice the length of the regular one, 

 was shot forth in a single day, Herschel says: 



''The projection of this ray, which was not seen either before or 

 after the daj" in question, to so enormous a length (nearly 100-) in a 

 single da}' conveys an impression of the intensity of the forces acting 

 to produce such a velocity of material transfer through space, such as 

 no other natural ])henomenon is capable of exciting. It is clear that 

 if we have to deal here with matter, such as we conceive it, viz., 

 possessing inertia, at all, it nuist be under the dominion of forces 

 incompara))ly more energetic than gravitation and (juite of a different 

 nature." 



And finally (p. 106): 



"There is beyond question some profound secret and m}\ster.v of 

 nature concerned in the phenomenon of their tails. In no respect is 

 the C[uestion as to the material it\' of the tail more forcibly pressed on 

 us for consideration than in that of the enormous sweep which it makes 

 round the sun in perihelio, in the manner of a straight and rigid rod, 

 in defiance of the law of gravitation, nay, even of the received law^s of 

 motion, extending (as we have seen in the comets of lOSU and 1813) 



