110 



SCEXEKY OP THE HEAVENS. 



the summer solstice that the earth attains its highest temperature, although its daily supply 



of solar influence is then actually diminishing. 

 The comet appears in the diagram at its 

 perihelion passage merely for the sake of 

 illustration, as in that part of its course it 

 was completely lost in the solar blaze. In the 

 other positions it was observed at the times 

 stated by Cassini, Newton, Halley, and 

 Flamstead. 



Cometary statisticians have compiled a 

 record of between six and seven hundred 

 appearances since the commencement of the 

 Christian era. But little dependence can be 

 placed upon this enumeration, as simple me 

 teors and such phenomena as new stars were 

 confounded in former times with true comets, 

 and instances of the re-appearance of the 

 same body are no doubt included in the 

 return. In about 200 instances, the orbits 

 have been ascertained with more or less 

 certainty. Of this number, forty appear to 

 have described ellipses ; seven hyperbolas ; 

 and a hundred and fifty parabolas. The 

 ellipse is an oval which admits of every 

 possible degree of eccentricity, or deviation 

 from the circle ; the hyperbola is an open 

 curve, the branches of which may be considered straight lines indefinitely divergent ; the 

 parabola is an open curve, the branches of which extend in a nearly parallel direction, never 

 converging. Consequently the elliptic comets are regular members of the solar system, 

 with periodic times, some of them never wandering beyond the planetary orbits, while the 

 hyperbolic and parabolic comets appear within its limits, depart, and return no more, unless 

 thrown by some disturbing force into a new path. 



The three features of nebulosity, nucleus, and tail, are usually assigned to cometary 

 bodies, but many are destitute of the latter appendage, and also without any clearly 

 defined nucleus. They appear as simple nebulosities, globular masses of vapour, having 

 no central condensation, through which the feeblest of the stars readily shine. Herschel 

 perceived a star of the sixth magnitude through the centre of the comet without nucleus 

 of the year 1795 ; and a star of the eleventh magnitude was perfectly distinguished by 

 Stmve through the middle of one of the short-period comets. Others present a nucleus 

 strongly defined, with surrounding nebulosity, the "horrid hair" of poetry. The 

 vapoury envelope is dim towards the central point, but suddenly becomes luminous 

 at some distance from it, so as to resemble a ring resting in equilibrium around a star, 

 like the ring of Saturn. The cometary nuclei often shine with a light as vivacious as 

 that of the planets, and exceed them in splendour upon nearing the sun. They vary 

 considerably in their diameters, but are in general very small The measurement of the 

 diameters of five given by Arago range between thirty-three miles and three thousand 

 two hundred. The external appearance of other comets exhibits the three features 

 combined, and these are remarkable objects, occasionally presenting a terrific aspect. 

 Immense spaces are sometimes covered by the luminous trains, or tails, as much as ninety 

 or a hundred degrees ; so that while the nucleus has been below the horizon, the train has 

 reached the zenith, stretching through an extent of nearly a hundred and fifty millions of 



