5ig ' LECTURE XLir. 



Georgian pknet are nearly perpendicular to the ecliptic; and some of tlieir 

 re/olutions are supposed to be rather retrograde than direct. 



Besides the bodies which revolve completely round the sun, within the li- 

 mits of our observation, there are others, of which we only conclude from 

 analogy, that they perform such revolutions. These are the comets; they 

 generally appear attended by a nebulous light, either surrounding them as a 

 coma, or stretched out to a considerable length as a tail; and they sometimes 

 seem to consist of such light only. Their orbits are so eccentric, that in 

 their remoter situations the comets are no longer visible to us, although at 

 other times they approach much nearer to the sun than any of the planets: 

 for the comet of ]6'80, when in its perihelion, was at the distance of only 

 one sixth of the sun's diameter from his surface. Their tails are often of 

 great extent, appearing as a faint ligbt, directed always towards a point 

 nearly opposite to the sun: it is quite uncertain of what substance they con- 

 sist; and it is difficult to determine which of the conjectures respecting them 

 can be considered as the least improbable; it is possible that, on account of 

 the intense cold, to which the comets are subjected in the greatest part of 

 their revolutions, some substances, more light than any thing we can imagine 

 on the earth, may be retained by them in a liquid, or even in a solid form, 

 until they are disengaged by the effect of the sun's heat: but we are still 

 equally at a loss to explain the rapidity of their ascent : for the buoyancy of the 

 sun's atmosphere cannot possibly be supposed to be adequate to the effect; and 

 on the whole there is, perhaps, reason to believe that the appearances are derived 

 from some cause, bearing a considerable analogy to the fluid, supposed to be 

 concerned in the effects of electricity. It is probable that the density of the 

 nucleus, or the body of the comet itself, is comparatively small, and its at- 

 traction for the tail consequently weak, so that it has little tendency to re- 

 duce the tail, even if it consists of a material substance, to a spherical form: 

 for since some comets have no visible nucleus at all, there is no difficulty in 

 supposing the nucleus, when present, to be of very moderate density, and 

 perhaps to consist of the same kind of substance as constitutes the tail or 

 coma, in a state of somewhat greater condensation. If, therefore, it should 

 ever happen to a planet to fall exactly in the way of a comet, of which there 

 is but very little probability, it is to be supposed that the inconvenience 

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