SECT, xxxvi.] TAILS OF COMETS. 407 



rapidity of its ascent cannot be i. ccounted for. It has been 

 attributed to that power in the sun which produces those 

 vibrations of ether which constitute light ; but as this theory 

 will not account for the cornet of 1824, which is said to have 

 had two tails, one directed towards the sun, and a very short 

 one diametrically opposite to it, our ignorance on this subject 

 must be confessed. In this case the repelling power of the 

 comet seems to have been greater than that of the sun. 

 Whatever that unknown power may be, there are instances 

 in which its effects are enormous ; for, immediately after the 

 great comet of 1680 had passed its perihelion, its tail was 

 100,000,000 miles in length, and was projected from the 

 comet's head in the short space of two days. A body of 

 such extreme tenuity as a comet is most likely incapable of 

 an attraction powerful enough to recall matter sent to such 

 an enormous distance; it is therefore, in all probability, 

 scattered in space, which may account for the rapid decrease 

 observed in the tails of comets every time they return to 

 their perihelia. Should the great comet of 1843 prove to 

 be the same with that of 166S, its tail must have diminished 

 considerably. 



It is remarkable that, although the tails of comets in- 

 crease in length as they approach their perihelia, there is 

 reason to believe that the real diameter of the head con- 

 tracts on coming near the sun, and expands rapidly on 

 leaving him. Hevelius first observed this phenomenon, 

 which Encke's comet has exhibited in a very extraordinary 

 degree. On the 28th of October, 1828, this comet was about 

 three times as far from the sun as it was on the 24th of 

 December; yet at the first date its apparent diameter was 

 twenty-five times greater than at the second, the decrease 

 being progressive. M. Valz attributes the circumstance to 

 a real condensation of volume from the pressure of the 

 ethereal medium, which increases most rapidly in density 

 towards the surface of the sun, and forms an extensive 

 atmosphere around him. It did not occur to M. Valz, how- 



