Comets in the Solar System. 19 



of Mercury and the Sun, ought nearly all to be observed from 

 the Earth : 1st, Because their angular velocity not being very 

 great, a few cloudy days would not be sufficient to transport 

 them from our hemisphere into the opposite one, in which the 

 earth's curve would hide them from our view ; and, 2dly, Be- 

 cause near the sun, and swimming, as it were, in its light, these 

 stars, though they possessed a physical constitution the least of 

 all favourable to it, must reflect a sufficient number of rays to 

 be easily perceptible. 



The comets which are included between the sphere of Mer- 

 cury and that of Venus, as seen from the earth, appear to move 

 more rapidly, and are conspicuously less brilliant, than those in- 

 cluded in the former list. All other things, therefore, being 

 equal, a smaller proportion of these ought to be seen. 



As to those comets whose perihelion distance diffi2rs but little 

 from a radius of the earth's orbit, we shall find that, besides be- 

 ing more feebly illuminated than those which traverse, for ex- 

 ample, the orbit of Mercury, in a ratio which, expressed in num- 

 bers, would exceed that of 16 to 100, it will also appear that, 

 near our globe, their apparent progress is usually extremely ra- 

 pid, and that, on this account, they will generally be visible only 

 for a few days ; and if during that period the sky be at all 

 cloudy, no signal of their passage will be at all observed. 



If, then, it should now be inquired why the table last given 

 enumerates so few comets beyond the orbit of Mars, it will be 

 sufficient to remark, in general, that these stars, whatever their 

 perihelion distance may be, cease to be visible from the earth so 

 soon as their course has transported them to a distance from the 

 sun equal to' three or four radii of the earth's orbit ; and, conse- 

 quently, those comets whose perihelion is found situated beyond 

 the orbit of Mars, must run their orbit without being perceived 

 from the earth ; at all events, when they are not possessed of a 

 size and a density, and consequently a brilliancy, which is alto- 

 gether extraordinary. 



I shall finally remark, more especially for the benefit of those 

 who are astonished at not finding in the table a single comet 

 whose perihelion extends beyond the orbits of Jupiter and Sa- 

 turn, that the comet of 1759, after its last appearance, sojourn- 

 ed for fivd whole years in the ellipsis which Saturn maintains, 



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