356 TAILS OF COMETS. SECT. XXXVI. 



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

 had two tails, one directed toward the sun, and a very 

 short one diametrically opposite to it, pur ignorance on 

 this subject must be confessed. In this case the repel- 

 ling 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 power- 

 ful 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 ob- 

 served 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 1668, its tail must have di- 

 minished 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 

 contracts on coming near the sun, and expands rapidly 

 on leaving him. Hevelius first observed this phenome- 

 non, which Encke's comet has exhibited in a very ex- 

 traordinary 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 vol- 

 ume from the pressure of the ethereal medium, which 

 increases most rapidly in density toward the surface of 

 the sun, and forms an extensive atmosphere around him. 

 It did not occur to M. Valz, however, that the ethereal 

 fluid would penetrate the nebulous matter instead of 

 compressing it. Sir John Herschel, on the contrary, 

 conjectures that it may be owing to the alternate con- 

 version of evaporable materials in the upper regions of 

 the transparent atmosphere of comets into the states of 

 visible cloud and invisible gas by the effects of heat and 

 cold ; or that some of the external nebulous envelops 



