Imperial Institute of France, 65 



blance with those of a comet observed in China 510 years 

 ago. This remark, if it could liave been verified, would 

 give the true nieasurcment of the revolution and of the 

 ellipsis of the comet ; but this knowledge is by its nature 

 very uncertain, when we have no other observations than 

 those of a single appearance. M. de Flaugergues, on go- 

 ing back to antiquity, found several comets, the appearances 

 of all Tvhicb differed by 310 years ; and they would give a 

 high degree of probability to his conjecture, if the indica- 

 tions of historians were not too vague to admit of the 

 orbit being calculated. We have therefore nothing certain 

 as yet on this head. Messrs. Bouvart, Gauss and Lin- 

 denau, who have also determined the orbit, think on the 

 contrary, that the period could not be less than 1000 or 

 1500 years, and that it might be much more. When we 

 have eight months cbservalions we shall perhaps be in a 

 little less uncertainty on this subject; but even this is very 

 problematical. The above co;iiet after all that has been 

 said of it had nothing in it more remarkable than any other 

 comet. After having determined the route which it ought 

 fo take, astronomers can only repeat what is printed in 

 every treatise of astronomy. But they ought not to con- 

 fine then)selves to this dry detal : we ought to have some 

 dissertations on the physical constitution of the comet, on 

 the nature and cause of that long tail, which in telescopes 

 looks like a veil attached to a woman's head, and which is 

 symniftrically imfolded on both sides in two opposite curves, 

 at first distant, but which end by approaching and being 

 melted into each other. Astronomers in this respect are 

 no further advanced than our ancestors. The explanation 

 which Newton gave of the tails of comets, accounts en a 

 large scale for the most remarkable phsenomena, i. e. for the 

 direction, which is always, with a slight variation in the 

 prolongation of the line which unites the centres of the 

 sun and of the comet, with a slight curvature which in- 

 clines it towards the place which the comet has quitted ; 

 but it is difficult thereby to account for the inclination of 

 the other branch in a contrary direction ; and^ihis phaino- 

 menon has been remarked by all astronomers. Wherefore 

 does tins tail, or that atmosphere of which the tail is the 

 prolongaiion, appear separated in every point from the 

 head or nucleus ? 



This obscure interval, which has been remarked in a 

 similar maiincT constantly, docs not take plate in all co- 

 mets ; but it is not without example, is the separation 

 real ? Is it an optical illusion, and, if it does take place, 



w hat 



