PHYSICAL CONSTITUTION OF COMETS. 



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| no'iced in the head of certain comets, where a stratum void of nebula has been 



> observed, interposed, as it were, between the denser portion of the head, or 

 [ nucleus, and the coma. It is analogous to the meteorological phenomenon of 



> a definite vapor plane, so commonly observed ; and in certain cases, may admit 

 [ of two or more alternations of nebula and clear atmosphere." 



Sir John offers a third supposition to account for the effects, by attributing 

 [ them to the ethereal medium surrounding the sun. 



" Fourier," says he, " has rendered it not improbable, that the region in 

 | which the, earth circulates has a temperature of its own greatly superior to 

 i what may be presumed to be the absolute zero, and even to some artificial de- 

 | grees of cold. I have shown, I think, satisfactorily, that if this be the case, 

 such temperature cannot be due simply to the radiation of the stars, but must 

 arise from some other cause, such as the contact of an ether, possessing itself 

 a determinate temperature, and tending, like all known fluids, to communicate 

 this temperature to bodies immersed in it. Now if we suppose the tempera- 

 ture of the ether to increase as we approach the sun, which seems a natural, 

 and indeed a necessary consequence, of regarding it as endued with the ordi- 

 nary relations of fluids to heat, we are furnished with an obvious explanation 

 of the phenomenon in question. A body of such extreme tenuity as a comet, 

 may be presumed to take very readily the temperature of the ether in which 

 it is plunged ; and the vicissitude of warmth and cold thus experienced, may 

 alternately convert into transparent vapor, and reprecipitate the nebulous sub- 

 stance, just as we see an increase of atmospheric temperature dissipate the 

 fog, not by abstracting or annihilating its aqueous particles, but by causing 

 them to assume the elastic and transparent state which they lose, and again 

 appear in fog when the temperature sinks." 



CONSTITUTION OF THE COMETS. 



The word comet is derived from a Greek word signifying hair, and hence 

 the name implies a hairy star. The nebulosity, or a sort of illuminated haze 

 which always appears around these bodies, is that from which the name was 

 probably taken. 



The head of the comet is the brightest part of the centre, usually supposed 

 to be a nucleus something like that of a planet ; but this is so enveloped in the 

 hair, or nebulosity, that it has never yet been satisfactorily ascertained whether 

 it be solid matter. 



A luminous train, varying in length, is frequently, though not always, attached 

 to these objects. It has been generally called the tail. Sometimes comets 

 have more than one of these appendages. 



THE NEBULOSITY. 



As the brightness of the nebulosity gradually fades away toward the edges, 

 there is sometimes a difficulty in measuring its bulk. Its form is generally 

 globular, and its light is often so faint that the comet can only be discovered by 

 telescopes. The diameter of the nebulous mass has been found to vary from 

 6,000 miles upward. The comets of 1795, 1797, 1798, and 1804, were sur- 

 rounded by a nebulosity which measured less than 7,000 miles in diameter. 



That many comets have no solid matter in the centre of the nebulosity is 

 proved by the fact that the smallest stars are often visible through them ; even 

 the ancients, without the aid of the telescope, ascertained this fact. Seneca 

 reported that stars were discoverable through comets, although he does not 

 distinctly state through what part of the comet they were seen. Sir William 



