ON COMETS. 105 



remote, that is to say, at the two extremities of their 

 elliptic orbits, or what are termed tteir perihelion and 

 aphelion. By far the great majority approach it at their 

 perihelion near enough to arrive within the earth's orbit 

 very many within that of Venus, or even of Mercury 

 and not a few attain an extreme proximity to the 

 actual surface of the sun, while on the other hand only 

 four or five among the vast number of recorded comets 

 (those of 1747, 1826, 1835, 1847) have failed to arrive 

 within twice the earth's distance, or within the orbits of 

 those small planets called asteroids ; and one only has 

 had a perihelion distance exceeding four times the earth's 

 distance (that of 1729), still falling short of the orbit of 

 Jupiter. Probably, however, a comet, which should 

 always remain outside of the latter planet's orbit, would 

 have no chance of ever being seen by us. As to the 

 extreme distances to which they recede from the sun, it 

 is only in comparatively few instances that it can be even 

 estimated their ellipses being in general so elongated 

 as to be undistinguishable from that extreme and limit- 

 ing form which is called a parabola, which never returns 

 into itself at all. The form of this curve is that which a 

 stone thrown into the air describes, or which a jet of 

 water thrown up obliquely by a smooth round pipe 

 assumes in the air, being very much curved or bent 

 about the point which is called the vertex, and less and 

 less so in the ascending and descending branches. 



(13.) Comets, we have said, are wild wanderers, and 

 despise beaten tracks. No way confined, as the planets 

 are, to move in planes nearly coincident with the ecliptic, 



