406 TAILS OF COMETS. [SECT, xxxvi. 



ture ; 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 ap- 

 peared in the years 1618, 1680, and 1769, had tails which 

 extended respectively over 104, 90, and 97 degrees 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 envelopes, 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 vapour, with little or no tail. As they ap- 

 proach 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 splendour. They then gradually 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 recorded 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 



