SECT. XXXV. TAILS OF COMETS. 377 



till they emerge from the sun's more vivid light that they assume 

 their full splendour. They then gradually decrease, their tails 

 diminish, and they disappear, nearly or altogether, before they 

 are beyond the sphere of telescopic vision. Many comets have 

 no tails, as, for example, Encke's comet. Those which appeared 

 in the years 1585, 1763, and 1682, were also without tails, 

 though the latter is recorded to have been as bright as Jupiter. 

 The matter of the tail must be extremely buoyant to pre- 

 cede a body moving with such velocity : indeed, the rapidity of 

 its ascent cannot be accounted 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 comet 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 con- 

 fessed. 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 enor- 

 mous ; for, immediately after the great comet of 1680 had passed 

 its perihelion, its tail was 100,000,000 miles in length, arid 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 in- 

 capable of an attraction powerful enough to recall matter sent to 

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

 scattered in space or absorbed by the zodiacal light or nebula 

 that surrounds the sun, 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 1668, its tail must have diminished considerably. 



It is remarkable that, although the tails of comets increase 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 ob- 

 served 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, 



