COMETARY INFLUENCES. 



attractive force. ]S"ow, by the general law of gravitation, that 

 attraction is always reciprocal, and it is certain that the comets 

 attract the planets as strongly as the planets attract them, and if 

 the masses of the comets were as great as those of the planets, 

 they would cause the planets to deviate from their accustomed 

 path as widely as the planets cause them to deviate. If, however, 

 we find, that while the deviation of the comets, in virtue of this 

 mutual attraction, is very great, that of the planets is extremely 

 small, the inference must be that the masses of the comets are 

 smaller than those of the planets, in exactly the proportion in 

 which the effect of the attraction on the planet is less than its 

 effect upon the comet. 



Now, in fact, it has been found that while the deviation of the 

 comets, due to the attractions of the planets, is very considerable, 

 that of the planets, of the satellites, and even of the planetoids 

 (the smallest bodies of the solar system), is so minute as to be 

 absolutely inappreciable by the most exact means of observation. 

 A case is even recorded in which a comet passed almost in contact 

 with the satellites of Jupiter, if, indeed, it did not pass among 

 these small bodies, yet its attraction upon them was so feeble as 

 to produce not the slightest observable effect upon their motions, 

 although the comet itself, by the attraction of the planet, was so 

 strongly affected that its orbit was completely changed. 



By such observations and calculations it has then been esta- 

 blished that, although the comets are masses of ponderable matter, 

 the quantity of matter composing each of them is incalculably less 

 than that of the smallest planet, primary or secondary, of the solar 

 system. 



These bodies are as remarkable for the vastness of their mag- 

 nitude, and the strangeness, variety, and mutability of their forms 

 as for the smallness of their masses. 



Comets in general, and more especially those which are visible 

 without a telescope, present the appearance of a roundish mass of 

 illuminated vapoisr or nebulous matter, to which is often, though 

 not always, attached a train more or less extensive, composed of 

 matter having a like appearance. The former is called the HEAD, 

 and the latter the TAIL of the comet. 



The tail is more significantly called the brush by Chinese astro- 

 nomers. 



The illumination of the head is not generally uniform. Some- 

 times a bright central spot is seen in the nebulous matter which 

 forms it. This is called the NUCLEUS. 



The nucleus sometimes appears as a bright stellar point, and 

 sometimes presents the appearance of a planetary disk seen through 

 a nebulous haze. In general, however, on examining the object 

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