106 ON COMETS. 



they cut across it at every possible angle, and, as nearly 

 as can be ascertained (with exception of one small class 

 of comets), quite indifferently as to the degree of their 

 inclination, or to the direction of the longer axes or long- 

 est dimensions of their orbits in space ; so that there is 

 no region of space, however situated either in direction 

 or distance from the sun, which a comet may not visit. 

 Neither do they conform to that other universal planet- 

 ary rule of circulation round the sun in one direction. 

 Retrograde comets, or those whose motion is opposite to 

 that of the planets, are as common as direct ones, or 

 those which conform to the planetary rule. Here again, 

 however, there is a small class in which a tendency to 

 conformity is exhibited, co-extensive with that above 

 noticed, which affects a certain proximity to the ecliptic. 

 But of this we shall have occasion to speak more at 

 large. 



(14.) It is only when all the particulars which determine 

 geometrically the situation and the form of the orbit of 

 a comet, its nearest distance from the sun, and the 

 direction in which it is moving, or what are called the 

 elements of its orbit, that it can be ascertained whether 

 it has ever been seen before, and whether we are to 

 expect ever to see it again ; and that its future course, 

 while it remains invisible, can be predicted with cer- 

 tainty. These elements are technically called 



1. T\\z perihelion distance, or nearest approach to the 



sun. 



2. The eccentricity of its ellipse, or whether .the orbit 



be sensibly a parabola. 



