On the Formation of the Tails of Comets. 107 



comet as seen on the 28th of February ; sometimes perhaps con- 

 vex. The whole tail is generally bent, so as to be concave to- 

 wards the regions of space which the comet has just left. This 

 curvature is most perceptible near the extremity of the tail, and 

 has in one instance (that of the comet of 1744) been observed to 

 be as great as 70° or 80°. The envelope of the head is not a 

 hemisphere, but approximates to the form of a hyperboloid, hav- 

 ing the nucleus in its focus, and its vertex turned towards the 

 sun. In some instances the nucleus is furnished with several 

 envelopes concentric with it ; and sometimes each of these is pro- 

 vided with a tail. Each of these several tails lying one within 

 the other, being hollow, may in consequence appear so faint along 

 its middle as to have the aspect of two distinct tails. A comet 

 that has in reality three separate tails, might thus appear to be 

 supplied with six, as was the comet of 1744. If the different en- 

 velopes were not distinctly separate from each other, then we 

 should have all the tails appearing to proceed from the same ne- 

 bulous mass. 



3. Variations in the form and dimensions of Comets. — It is 

 well known that both the head and tail of a comet are subject to 

 great changes in their apparent dimensions, and frequently some 

 slight changes of form, during the period of its visibility, which 

 have an obvious relation to the distance of the comet from the sun. 

 These changes cannot be attributed to the variations in the amount 

 of light received from the sun ; for, in the first place, they are too 

 irregular, and, in the next place, the length of the tail, as well as 

 the intrinsic lustre of the whole comet, continue to increase until 

 some time after the perihelion passage. The nebulous envelope 

 of the head moreover, increases in its dimensions as the comet 

 departs from the sun. It follows, therefore, that a comet is not 

 a body of invariable size, like the planets, and that the tail is ac- 

 tually formed out of a portion of the matter of the head, as it ap- 

 pears to be ; and this by the operation of some cause whose ac- 

 tion depends upon the distance of the head from the sun. 



Such are most of the facts appertaining to the physical consti- 

 tution of the cometary bodies which will be employed in the 

 discussion upon which I propose to enter. Their importance in 

 the present connection, must be my apology for occupying the 

 pages of a Journal of Science with the recital of facts, for the 

 most part, long since established. 



