SECT, xxxvi.] TAILS OF COMETS. 405 



from electricity, or even causes with which we are un- 

 acquainted. The envelopes surrounding the nucleus of the 

 comet on the side next to the sun, diverge on the opposite 

 side, where they are prolonged into the form of a hollow 

 cone, which is the tail. Two repulsive forces seem to be 

 concerned in producing this effect ; one from the comet and 

 another from the sun, the latter being the most powerful. 

 The envelopes are nearer the centre of the comet on the 

 side next to the sun, where these forces are opposed to one 

 another; but on the other side the forces conspire to form 

 the tail, conveying the nebulous particles to enormous 

 distances. 



The lateral edges of the tail reflect more light than the 

 central part, because the line of vision passes through a 

 greater depth of nebulous matter, which produces the effect 

 of two streams somewhat like the aurora. Stars shine with 

 undiminished lustre through the central part of the tail, 

 because their rays traverse it perpendicularly to its thick- 

 ness; but, though distinctly seen through its edges, their 

 light is weakened by its oblique transmission. The tail of 

 the great comet of 1811 was of wonderful tenuity; stars 

 which would have been entirely concealed by the slightest 

 fog were seen through 64,000 leagues of nebulous matter 

 without the smallest refraction. Possibly some part of the 

 changes in the appearance of the tails arises from rotation. 

 Several comets have been observed to rotate about an axis 

 passing through the centre of the tail. That of 1825 per- 

 formed its rotation in 20^ hours, and the rapid changes in 

 the luminous sectors which issued from the nucleus of 

 Halley's comet in all probability were owing to rotatory 

 motion. 



The two streams of light which form the edges of the 

 tail in most cases unite at a greater or less distance from the 

 nucleus, and are generally situate in the plane of the orbit. 

 The tails follow comets in their descent towards the sun, but 

 precede them in their return, with a small degree of curva- 



