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PHYSICAL CONSTITUTION OP COMETS. 



are circular, or even nearly so. Some are ovals of various eccentricity. Some 

 are either parabolas, or ellipses of such extreme eccentricity as to be undistin- 

 guishable from parabolas by any observations we have been enabled to make 

 upon them. Others, again, seem to move in hyperbolas. 



The magnitudes of the planetary orbits increase regularly, according to a 

 certain harmonious proportion. No order or regularity is discoverable among 

 the magnitudes of the cometary orbits. 



The orbits of comets are not confined to the plane of the ecliptic : they are 

 found to be at every possible angle with it from to 90. Arago has exam- 

 ined the position of the orbits of a great number of comets, and has found that 

 an equal number move at every inclination with the ecliptic. 



Unlike planets, comets do not move in one uniform direction round the sun. 

 Some move in the same direction as the earth, and some in the opposite direc- 

 tion. There are about as many retrogade as direct. 



Such are the chief circumstances which distinguish the motions of the com- 

 ets from those of the planets. 



NUMBER OF COMETS. 



The determination of the number of comets connected with our system is a 

 question which, although not admitting of a demonstrative solution, may be 

 solved upon grounds of a high degree of probability ; and it is one of so much 

 interest, that we are induced here to lay before our readers the views of M. Ara- 

 go and others on this point. 



The total number of distinct comets, whose paths during the visible parts of 

 their course had been ascertained up to the year 1832, was one hundred and 

 thirty-seven. In order to discover whether bodies of this nature prevail more 

 in any particular regions of space than in others whether, like the planets, 

 they crowd into a particular plane, or are distributed through the universe with- 

 out any preference of any one region to any other it was necessary to exam- 

 ine and compare the paths of these hundred and thirty-seven bodies. After a 

 close examination of the planes of their orbits with respect to that of the earth, 

 it appears that the numbers inclined at various angles, from to 90, is pretty 

 nearly the same. Thus, at angles between 80 and 90 there are fifteen com- 

 ets ; while at angles between 10 and 20 there are thirteen ; and between 30 

 and 40 there are seventeen. Again, the points where they pass through the 

 plane of the earth's orbit are found to be uniformly distributed in every direc- 

 tion around the sun. The points where they pass nearest to the sun are like- 

 wise distributed uniformly round that body. Their least distances from the sun 

 also vary in such a manner as leads to the supposition of their uniform distri- 

 bution through space. Thus, if we suppose a globe, of which the sun is the 

 centre, to pass through the orbit of Mercury, so as to enclose the space round 

 the sun, extending a distance on every side equal to the distance of Mercury, 

 thirty of the ascertained comets, when at their least distance from the sun, pass 

 within that globe. Between that globe and a similar one through the orbit of 

 Venus, forty-four comets pass under like circumstances. Between the latter 

 globe and a like one through the orbit of the earth, thirty-four pass. Between 

 the globe through the orbit of the earth and one through the orbit of Mars, 

 twenty-three pass ; and between the latter and a globe through the orbit of Ju- 

 piter, six pass. No comet has ever been visible beyond the orbit of Jupiter. 

 It must be here observed, that beyond the orbit of Mars it is extremely difficult 

 to discern comets ; and this may account for the comparatively small number 

 of ascertained comets which do not come nearer to the sun than that limit A 



