On the Formation of the Tails of Comets. 125 



together, the motion relative to the head will obviously be some- 

 what different from what it otherwise would be, in consequence 

 of the change of direction of the sun's force produced by the 

 orbitual motion. 



Making use of certain data mentioned in a former part of 

 this article, I find that the sides of the tail of the recent comet 

 diverged from each other on the third of March, as if they 

 came from a point but little more than 100,000 miles from the 

 head. The comet of 1744 presents a case of still greater di- 

 vergence. If, therefore, the matter of which the tail is made up 

 be supposed to be detached from the marginal parts of the nebu- 

 losity by the repulsive force of the sun, there must sometimes, if 

 not always, be some cause in operation tending to increase the 

 divergence of the sides of the tail due to the separation of the 

 lines of direction of the force. This can apparently be found 

 in a rotation of the head. The effect of such a rotation would 

 be to make the particles revolve around the axis of the tail, at 

 the same time that they are darting along in the direction of its 

 length, and thus to recede continually from the axis ; as there 

 would be but a small quantity of matter within their orbits to 

 oppose by its attraction the centrifugal force consequent upon 

 the motion of revolution. But is there any evidence of a rota- 

 tion ? To this question 1 make answer, that in the first place, 

 since all the other heavenly bodies, so far as known, have this 

 species of motion, there is a fair presumption that the cometary 

 bodies have it likewise. In the next place, certain observed 

 changes in the relative position of the multiple tails of some 

 comets, and of the sides of the single tail of others, have ren- 

 dered it almost certain that the tails of some of these bodies had 

 a rapid rotation about their axes. The comets of 1769, 1811, 

 and 1825, may be cited as examples. A rotatory motion of Hal- 

 ley's comet at its last appearance in 1835, was inferred from the 

 rapid changes noticed in the situation of certain luminous streams, 

 in the form of circular sectors, seen to issue from the nucleus on 

 the side towards the sun. The observations upon the recent 

 comet made at the Cape of Good Hope, (see Amer. Almanac for 

 1844, p. 95,) would seem to indicate the existence of the same 

 motion in the tail, or rather tails, of this comet also. To produce 

 a revolution of the particles of the tail around the axis, it is not 

 necessary that the head should rotate about the prolongation of 



