354 TAILS OF COMETS. SECT. XXXVI. 



Indeed, comets in general are subject to sudden and 

 violent convulsions in their interior, even when far from 

 the sun, which produce changes that are visible at enor- 

 mous distances, and baffle all attempts at explanation, 

 probably arising from electricity, or even causes with 

 which we are unacquainted. The envelops 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 envelops 

 are nearer the center of the comet on the side next to 

 the sun, where these forces are opposed to one an- 

 other; but on the other side the forces conspire to 

 form the tail, conveying the nebulous particles to enor- 

 mous 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 thickness ; but though distinctly seen through its 

 edges, their light is weakened by its oblique transmis- 

 sion. The tail of the great comet of 1811 was of won- 

 derful tenuity ; stars which would have been entirely 

 concealed by the slightest fog, were seen through 64,000 

 leagues of nebulous matter without the smallest refrac- 

 tion. Possibly some part of the changes in the appear- 

 ance of the tails arises from rotation. Several comets 

 have been observed to rotate about an axis passing 

 through the center of the tail. That of 1825 performed 

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

 luminous sectors which issued from the nucleus of Hal- 

 ley'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 

 toward the sun, but precede them in their return, with 



