On the Formation of the Tails of Comets. 123 



occurred to some of the numerous theorists who had speculated 

 upon the mysteries of the luminous appendages of comets. A 

 communication setting it forth with some degree of detail, and 

 offering some arguments in support of it, together with others 

 against the view of the matter generally entertained, was after- 

 wards read before the American Philosophical Society at the 

 celebration of the hundredth anniversary of the Society in May 

 last ; a brief abstract of which has since been published in the 

 account of the proceedings had on that occasion. Circumstances 

 beyond my control had hitherto prevented me from making such 

 examinations of astronomical works as to satisfy myself whether 

 the conception which I had formed of the tails of comets was 

 altogether new, and I, accordingly, did not, on this occasion, pre- 

 sent any claim to be the originator of it. I have since found that 

 it is a feature of Olbers' more complete theory, as modified by 

 Bessel.* With the fact that Olbers conceived a portion of the 

 matter of the nucleus to be driven off by a repulsion residing in 

 the nucleus I had long been acquainted, but no explanation of 

 the theory that I had met with, represented it as conceiving 

 matter to be flowing off continually to an indefinite distance. 

 Most of the theories that have been promulgated, contemplate 

 the matter of which the tail consists, as having been expelled 

 from the head by some force taking effect in a direction from the 

 sun. There was nothing in the mere assertion of a new operative 

 force to suggest the idea of a perpetual emission, such that there 

 should be at no time any physical bond of connection between 

 the head and tail of the comet ; any more than there had been in 

 the assertion of other forces having the same general tendency. 

 I have made this brief historical statement, that it might not be 

 supposed that I had on any occasion urged unfounded claims to 

 priority of discovery. 1 now proceed to the consideration of the 

 matter in hand ; and will begin with a more precise and detailed 

 explanation of the theory. First, then, I conceive that the tail of 

 a comet is made up of particles of matter continually flowing away 

 at a very rapid rate from the head into free space, and that at any 

 one instant we see the collection of all the particles that have 

 been emitted during a certain previous interval. According to 

 this, at the end of any such interval we are looking at an entirely 



* For the outline of this theory, see Vol. xlv, p. 188, of this Journal. 



