A RWG OF WORLDS. 



473 



as fragments of one which had burst, Olbers per- 

 ceived that there was a certain region of the 

 heavens where he would have a better chance of 

 discovering other fragments than anywhere else. 

 Every fragment after the explosion would have a 

 path passing through the place where the explo- 

 sion occurred. For the place of explosion, being 

 the spot from which each fragment started, would 

 of necessity be a point along each fragment's 

 future track. The fragments, be it understood, 

 would not return simultaneously to that spot. 

 Those which had been driven forward (more or 

 less) would have their period of circulation length- 

 ened, those which had been driven backward 

 would have their period shortened ; these last 

 then would return to the scene of the outburst 

 sooner than the former, and in point of fact no 

 two would return simultaneously to that place 

 unless, by some utterly improbable chance, they 

 had been hastened or retarded in exactly the same 

 degree. But all would pass through that spot 

 for many centuries after the terrible catastrophe 

 which had scattered them on their various paths. 

 If the region of the heavens toward which that 

 spot lay could be determined, then, the careful 

 observation of that region probably would soon 

 be rewarded by the discovery of other fragments. 

 Moreover, the region exactly opposite to it would 

 be similarly suitable for the search after these 

 small bodies ; for though their paths would not 

 all pass through a point exactly opposite the scene 

 of the explosion, these paths would all pass 

 through the prolongation of a line drawn through 

 the sun from that place. This is easily seen. 

 Every planet has its own plane of motion, in 

 which plane the sun necessarily lies ; if, then, we 

 know any one point of a planet's path, we know 

 that the line joining the sun and that point lies in 

 the plane of the planet's motion, and if extended 

 beyond the sun must cross the planet's track. 



Olbers then set himself the task of carefully 

 observing two parts of the heavens, one being the 

 place where the tracks of Ceres and Pallas ap- 

 proached each other nearest, the other being the 

 place directly opposite to this. One point is to 

 be noticed as essential to Olbers's faith in the 

 success of his method of search. In his day it 

 was generally believed that many centuries had 

 not passed since the planets had been set moving 

 on their respective paths. According to this view 

 the catastrophe by which Ceres and Pallas and 

 the fragments yet to be discovered had been sent 

 on their new courses, could not have occurred so 

 long ago that the paths of the fragments had been 

 materially displaced from their original position. 



If, on the other hand, millions of years might 

 have elapsed since the catastrophe happened, 

 there would have been little room for hoping that 

 the actual paths of the fragments would have re- 

 tained any trace of the peculiarity we have de- 

 scribed. It was somewhat fortunate for science 

 that Olbers had full faith in the doctrine that 

 the date of the catastrophe could not be more 

 than four or five thousand years before his time, 

 and that therefore he observed the two regions 

 of the heavens indicated by the explosion theory 

 with unwearying assiduity for many months. He 

 also persuaded Harding, of Lilienthal, to pay spe- 

 cial attention to these two regions ; one near the 

 northern wing of the Virgin, the other in the 

 constellation of the Whale. 



At length, on September 4, 1S04, the search 

 was rewarded with success ; the planet called 

 Juno being discovered by Harding in that part of 

 the Whale which Olbers had indicated. Olbers 

 did not cease from the search, however, but con- 

 tinued it for thirty months after Harding's success, 

 and five years after his own discovery of Pallas. 

 At length, on March 28th, the fifth anniversary of 

 this discovery, Olbers detected Vesta, the only 

 member of the family of asteroids which has ever 

 (we believe) been seen with the naked eye. 



For some reason astronomers seem to have 

 been satisfied with this fourth fragment of Ol- 

 bers's hypothetical planet. The search was not 

 resumed for twenty-three years. Then Hencke, 

 an amateur astronomer of Driessen, in Germany, 

 commenced a search destined to meet with no 

 success until more than fifteen years had elapsed. 

 We shall return presently to the discovery of the 

 fifth asteroid by Hencke. We must first, how- 

 ever, consider the interesting questions raised by 

 astronomers, after the discovery of Vesta, upon 

 the theory of Olbers that the asteroids are frag- 

 ments of an exploded planet. 



Lagrange, in 1814, examined the theory mathe- 

 matically, inquiring what degree of explosive force 

 would be necessary to detach a fragment of a 

 planet in such sort that it would not return, but 

 travel thereafter on an orbit of its own around 

 the sun. We have not by us the result of his 

 researches except as they are given in Grant's 

 " Physical Astronomy," as follows : 



" Applying his results to the earth, Lagrange 

 found that if the velocity exceeded that of a can- 

 non-ball in the proportion of 121 to 1, the fragment 

 would become a comet with a direct motion ; but 

 if the velocity rose in the proportion of 156 to 1, 

 the motion of the comet would be retrograde. If 

 the velocity were less than in either of these cases, 



