S*cr. XXXVI. TAILS OF COMETS. 365 



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 generally of enor- 

 mous 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, and 1769, had tails 

 which extended respectively over 104, 90, and 97 de- 

 grees of space. Consequently, 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 envelops, and inclosing one 

 another with intervals between ; the lateral edges of 

 these cones would give the appearance 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 vapor 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 till they emerge from the sun's more vivid light 

 that they assume their full splendor. They then grad- 

 ually decrease, their tails diminish, and they disappear 

 nearly or altogether before they are beyond the sphere 

 of telescopic vision. Many comets have no tails, as for 

 example Encke's comet, and that discovered by M. Biela, 

 both of which are small and insignificant objects. The 

 comets which appeared in the years 1585, 1763, and 

 1682, were also without tails, though the latter is re- 

 corded to have been as bright as Jupiter. The matter 

 of the tail must be extremely buoyant to precede a body 

 moving with such velocity ; indeed the rapidity of its 

 ascent cannot be accounted for. It has been attributed 

 to that power in the sun which produces those vibrations 

 of ether which constitute light : but as this theory will 



