376 TAILS OF COMETS. SECT. XXXV. 



perpendicularly to its thickness ; but, though distinctly seen 

 through its edges, their light is weakened by its oblique trans- 

 mission. 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 pass- 

 ing through the centre 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 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 curvature ; their apparent 

 extent and form vary according to the positions of the orbits with 

 regard to the ecliptic. In some cases the tail has been at right 

 angles to the line joining the sun and comet. The curvature is 

 in part owing to the resistance of the ether, and partly to the 

 velocity of the comet being greater than that of the particles at 

 the extremity of its tail, which lag behind. The tails are gene- 

 rally of enormous lengths ; the comet of 1811 had one no less 

 than a hundred millions of miles in length, and those which 

 appeared in the years 1618, 1680, arid 1769, had tails which ex- 

 tended respectively over 104, 90, and 97 degrees of space. Con- 

 sequently, when the heads of these comets were set, a portion of 

 the extremity of their tails was still in the zenith. Sometimes 

 the tail is divided into several branches, like the comet of 1744, 

 which had six, separated by dark intervals, each of them about 

 4 broad, and from 30 to 44 long. They were probably formed 

 by three hollow cones of the nebulous matter proceeding from the 

 different envelopes, and enclosing one another, with intervals 

 between ; the lateral edges of these cones would give the appear- 

 ance of six streams of light. The tails do not attain their full 

 magnitude till the comet has left the sun. When comets first 

 appear, they resemble round films of vapour, with little or no 

 tail. As they approach the sun, they increase in brilliancy, and 

 their tail in length, till they are lost in his rays ; and it is not 



