COMETS, THE SOLAR CORONA, AND THE AURORA. 45 



analogous to that which we sometimes see produced by a 

 very small insect on the surface of still water. We see a 

 dark spot, and on looking closer we find a small fly or 

 moth flapping its wings and creating a disturbance which 

 was visible before the insect which produced it. 



There is nothing else that we can conceive their tails to 

 be ; so that they must be one or the other of these two 

 things, — either 



(i) Material appendages of the nucleus, whether the 

 material be limited to the illuminated tail or surround the 

 comet on all sides — or 



(2) Matter which exists independently of the comet, 

 and on which the comet exerts such a physical influence 

 as to render it visible. 



Respecting the composition of these bodies Sir John 

 Herschel says : — ' ' There is beyond question some profound 

 secret and mystery of nature concerned in the phenomenon 

 of their tails. Perhaps it is not too much to hope that 

 future observation, borrowing every aid from rational 

 speculation, grounded on the progress of physical science 

 generally (especially those branches of it which relate to 

 the setherial or imponderable elements), may ere long 

 enable us to penetrate this mystery, and to declare 

 whether it is matter in the ordinary acceptation of the 

 term that is projected from their heads with such extra- 

 vagant velocities, and if not impelled at least directed in 

 its course by reference to the sun a as point of avoidance. 

 In no respect is the question as to the materiality 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 

 laws of motion, extending (as we have seen in the comets 

 of 1680 and 1843) fr° m near the sun's surface to the 

 earth's orbit, yet whirled round unbroken : in the latter 



