128 SCENERY OF THE HEAVENS. 



the system ? Are they inherently luminous, or dependent upon the solar glory, shining 

 like the planets by virtue of his light ? Have they any terrestrial influence ? Is there a 

 chance of our globe coming into actual collision with them; and supposing collision, what 

 would be its probable effects ? Upon most of these points we have no certain knowledge. 

 Herschel and Schroeter thought the comet of 1811 a self-luminous body, but in opposition 

 to this opinion Cassini is quoted as having descried the comet of 174-i showing a phase. 

 On the very day, says Arago, that any comet shall appear with a distinct phase, all doubts 

 will have ceased. At present however no satisfactory evidence is possessed of such 

 an appearance being observed. Upon the question of physical constitution, it is pretty 

 certain that the great majority of these bodies, and most probably all of them, are 

 entirely gaseous simple aggregations of vapour. The evidence to this effect is various. 

 The comet of 1770 passed twice through the system of Jupiter; and calculation shows, 

 that had it been -^ of one of the satellites in mass, it would have sensibly affected that 

 system. Yet there was not the slightest derangement of the planes of motion, or of the 

 periods of revolution, by its intrusion among the satellites. The same body also passed 

 at that time at no very great distance from the earth. In fact it approached us nearer 

 than any other that has visited our terrestrial sky. Had it possessed a quantity of solid 

 matter equal to that of the earth, it would then have shortened the length of our year by 

 one ninth of a day ; or had it been ^Vo" of the earth in mass, it would have appreciably 

 altered its length to a degree that must long ago have been observed. But not the least 

 perturbation was caused by its close proximity. These are sufficient proofs of the 

 smallness of its mass, even allowing it to have had any solid matter at all, which may be 

 reasonably suspected. Through the very centre of Biela's comet in 1832 a group of stars 

 of the sixteenth magnitude was very distinctly seen by Sir John Herschel. While 

 admitting that many comets are mere agglomerations of vapour, some hold to the opinion 

 that where there is a nucleus remarkable for its vivacity of light, there is a solid and 

 opaque body. But several facts declare against this supposition. Instances have 

 occurred of stars being visible through a strongly defined nucleus. In 1618, the nucleus 

 of the comet of that year is described as having dissolved into several detached parts ; 

 that of 1661 observed by Hevelius, changed also from a globular figure, and entirely 

 disappeared; and the most extraordinary transformations are suddenly effected in 

 the constitution of these objects. It is most probable that a comet is altogether a 

 gaseous body, and has no solid matter whatever. Sir John Herschel remarks, that 

 " whenever powerful telescopes have been turned on them, they have not failed to dispel 

 the illusion which attributes solidity to that more condensed part of the head, which 

 appears to the naked eye as a nucleus ; though it is true that in some a very minute 

 stellar point has been seen, indicating the existence of a solid body." Mr. Airy also 

 states, that " on the physical constitution of comets we have learnt nothing, except that 

 they appear to be wholly gaseous." 



These views of the constitution of cometary bodies show the fallacy of apprehending 

 those consequences from a shock with them, of which terrific pictures have been drawn, 

 and the impossibility of those events being produced by collision, which have been assigned 

 to it, such as the deluge of Noah, the depression of the Caspian Sea and its neighbourhood, 

 with the formation of the small telescopic planets out of a comet-stricken orb. " It is easy 

 to represent," says Laplace, " the effect of such a shock upon the earth ; the axis and 

 motion of rotation changed ; the waters abandoning their ancient position to precipitate 

 themselves towards the new equator ; the greater part of men and animals drowned in a 

 universal deluge, or destroyed by the violence of the shock given to the terrestrial globe ; 

 whole species annihilated ; all the monuments of human industry reversed ; such are the 

 disasters which a shock of a comet would produce. "We see then," he observes, referring 



