ASTRONOMY AND METEOROLOGY. 387 



The planets, and among them our earth, must again and again have trav- 

 ersed unharmed the tails of comets. In October last, the debris of the mag- 

 nificent train of the comet which has just disappeared from our western 

 skies, swept over the region occupied by the earth a few weeks earlier. In- 

 stances of more immediate proximity are of too common occurrence to 

 allow us to suppose that we are always to escape an actual collision; but it 

 is inconceivable that any disastrous consequences could ensue to our earth 

 or its inhabitants, any more than from contact with sunlight, or with the 

 ether of the planetary spaces. 



A second characteristic is that of internal condensation. All comets pre- 

 sent this in a greater or less degree. Most of them have a minute stellar 

 point, called the nucleus, which occupies the position of maximum density. 

 There are others in which this latter feature is wholly wanting. But the 

 number, in which it cannot be detected w T ith a powerful telescope, is much 

 smaller than has commonly been supposed. This centre of condensation, or 

 brightest point, is, with rare exceptions, placed on the side which is nearest 

 to the sun. It is always, however, very close to the centre of gravity, as is 

 proved by the fact of its motion about the sun, in accordance with the law 

 of gravitation. 



The nucleus itself is a minute point compared with the immense volume 

 of light-giving substance, of which it is the controlling centre. Whether it 

 is solid or not, is still undecided. As far as the eye alone is to be trusted, 

 there are comets as truly solid as the planets or stars themselves. In size 

 and weight, however, the true nuclei, apart from their surrounding nebulos- 

 ity, are probably quite small, measured by the standard of the larger plan- 

 ets. Still, it is possible that there may have been instances in which the 

 mass of these bodies has been comparable with that of the earth, and yet, 

 they may have completed their circuit around the sun, leaving no appreci- 

 able trace of their disturbing influence the only sure test by which their 

 mass could be detected. The evidence, from the fact that the smaller stars 

 shine freely, even through the most condensed portions of the comets, ad- 

 duced by astronomical writers in proof of their transparency, and, by infer- 

 ence, of their extreme tenuity and lightness, has a certain value when ap- 

 plied to the class of feeble telescopic comets, but is scarcely applicable to one 

 like that of the present year, which overpowered all but the brightest stars 

 in the neighborhood of the nucleus by its superior brilliancy. 



The feature next in importance to the nucleus is the train, or tail, as it is 

 usually called (although often preceding the nucleus in its motion), pro- 

 jected at an immense distance from it, and usually, although by no means 

 invariably, in a direction opposite to that of the sun. The agency of the 

 nucleus in the formation of the train, but still more in the subsequent con- 

 trol which it retains over it, is one of the most curious phenomena presented 

 in nature. Often several of these appendages are seen radiating at once 

 from the same nucleus. The greatest variety in curvature of outline, length, 

 brilliancy and other peculiarities, is presented by different comets, or by the 

 same one in different parts of its course. The portions near the axis are 

 usually darker than the edges, giving at times the appearance of a division 

 with a stream of light on either side. 



The larger bodies of this class exhibit a wonderful complication of phe- 

 nomena in the region contiguous to the nucleus. Of these the most prom- 

 inent are the interposition between the nucleus and the sun of one or more 

 well-defined and rounded screens, or caps of dense nebulosity, called envel- 



