86 HISTORY OF ASTRONOMY. 



family likeness running through the entire group, and it 

 naturally suggests the idea of a common origin. This 

 idea, as has been already stated, occurred to the mind of 

 Olbers after the discovery of the second asteroid, and led 

 to his celebrated theory that all these bodies originally 

 constituted a single planet which had been broken into 

 fragments by the operation of some internal force. Have 

 we any means of testing the soundness of this theory ? 



If the earth should be broken into fragments by the 

 operation of some internal force (such, for example, as 

 that which causes the eruption of a volcano,) the frag- 

 ments might be projected in various directions, and with 

 very unequal velocities ; but each would describe an 

 ellipse of which the sun would occupy one of the foci 

 if we except the extreme but possible case of a fragment 

 projected with such a velocity as to carry it beyond the 

 limit of the sun's attraction. Leaving out of view the 

 disturbance arising from the mutual attraction of the 

 planets, which produces only minute effects, each frag- 

 ment would continue to describe the same ellipse in its 

 successive revolutions about the sun ; in other words, 

 these ellipses would all have a common point of intersection. 

 The same conclusion must hold true for the asteroids, 

 according to the theory of Olbers. The question then 

 arises, have the orbits of the asteroids a common point 

 of intersection ? A single glance at the above diagram 

 will settle this question in the negative. But Olbers 

 replies that the orbits of the planets are disturbed by 

 their mutual attractions. These orbits should originally 



