A CAPTIVE COMET. 355 



mass of the comet swept over the surface of Jupiter itself, and 

 that we had here a true collision between the two bodies. The 

 comet struck the planet a glancing blow, as it were. As is usual 

 in all collisions, the weaker body suffered : the comet was broken 

 into three parts, while Jupiter was unharmed. 



When the comet had passed far enough away from Jupiter, so 

 that the sun had regained its supremacy, the motion was again 

 referred to the sun as fixed point, and the tedious process of trac- 

 ing the comet's history continued, step by step. Tracing thus 

 backward the path of this minute body, we find that it leads to 

 the spot where Lexell's comet disappeared in 1779. Either two 

 comets can occupy the same space at the same time, or the comets 

 of 1770 and 1889 are one and the same. 



We have thus seen something of the laborious process by 

 which, starting with a few observed positions of a body in 18S9, 

 we can trace the path it has traveled for one hundred and seven 

 years ; how we can show its identity with a comet seen in 1770. 

 Of the path itself but little has yet been said. It is interesting. 

 And as it is always easier to trace a succession of events in the 

 order in which they occur than it is to reverse the order of time, we 

 will start with the first recorded public appearance of the comet, 

 in 1770, and give a brief sketch of its erratic course through the 

 heavens since that moment — though, remember, this path was 

 discovered by tracing the body backward along its course. 



Look again at the diagram. In the summer of 1770 the comet 

 was seen moving along the small dotted curve in the region very 

 close to the smallest circle in the diagram, which represents the 

 orbit of the earth. It disappeared from view and passed outward 

 along this dotted curve, making one complete revolution, returning 

 in 1775 to the point where it was first seen. During these few years 

 the earth had also been traveling its yearly path around the sun, 

 and it so happened that in this latter year (1775) the earth had 

 moved into a different position in its orbit, so that the sun was 

 directly between it and the comet. The comet was therefore not 

 then seen. Onward went the comet along this dotted path, until, 

 in 1779, it had reached the outermost point, when it encountered 

 Jupiter. The effects of this appulse were very marked as regards 

 the comet : it was pushed completely out of its path and set 

 moving in an immense ellipse, the one that extends far out to the 

 left in the diagram. From five and a half years its period had 

 been changed to about thirty-four years. In this large path this 

 captive body moved without any extraordinary incident for sixty- 

 seven years, or until 184(3. During this time it had traveled twice 

 around the curve, and it was fairly started on its third trip, when 

 Saturn took a hand in the game and altered its path considerably, 

 extending the ellipse to one of forty-seven years period. On it 



