40 On the Tails of Comets. 



greater orless in proportion as the union of rays is more or less 

 distant from the comet. It is manifest that if a ray of h'ght could 

 be traced during its entire course from the sun to a planet, it would 

 present a similar phenomenon, equal in degree if the motion of 

 the planet were swift as that of a comet. The comets of Biela 

 and Encke have no tails, nor is there, strictly speaking, a nucleus 

 in either. That of Encke, during the long period in 1828, when 

 its position was so favorable to observation, had the appearance of 

 a mere film of vapor, nearly circular but not well defined, and no 

 central, stellar point could be detected with the telescopic power 

 which I employed on that occasion. In fact, all the phenomena 

 of the tails of comets appear to be so well explained by this theory 

 that I can not doubt its truth, although nothing like demonstration 

 accompanies it. There are indeed optical difficulties which I 

 have been unable to overcome : no one however which may not 

 be fairly attributed to our ignorance of the particular physical 

 constitution of these bodies. It is no small confirmation of the 

 truth of this explanation of the tails of comets, that there is not 

 the slightest evidence, worthy of confidence, that the earth which 

 we inhabit has ever been sensibly affected by a visitation from 

 these enormous appendages, while the chance of collision between 

 the earth and the nucleus of a comet, properly so called, is ex- 

 ceedingly small ; yet when we reflect upon the number of comets 

 belonging to our system, the hundreds that range within the 

 earth's orbit, that their paths have every possible inclination to 

 the ecliptic, that these immensely extended trains, projected in 

 a direction /rom the sun, describe an inconceivable sweep when 

 they are encompassing the sun in the region of their perihelion ; — 

 I say in view of these circumstances, it is difficult to avoid the 

 conjecture, nay, it is exceedingly probable that these appendages, 

 in very many instances, have brushed across the surface of our 

 planet, harmlessly and unperceived. 



I submit this theory (if indeed it is entitled to that name) to 

 the consideration of the scientific, having no point to gain, no 

 wish to gratify, but the promotion of science and the progress of 

 truth, and if insuperable objections to it are raised, and my rea- 

 soning should prove fallacious, there will be at least one valuable 

 result, that of showing what the tails of comets are not; more- 

 over, it may be the humble means of exciting further inquiry on 

 this interesting topic. 



Nantucket, 10th mo., 1st, 1839. 



