ZONE OF PLANETS BETWEEN MARS AND JUPITER. 89 



admit that such an occurrence could only have taken 

 place myriads of years ago. 



The preceding difficulties, however, are small in com- 

 parison with another which remains to be stated. The 

 orbit of Hygeia completely incloses the orbit of Flora 

 (and indeed several other orbits), and would still inclose 

 them, although the greater diameter of each of them 

 were revolved through- an entire circumference, since the 

 least distance of Hygeia from the sun exceeds the greatest 

 distance of Flora. The same is true of Themis, as com- 

 pared with Flora and several other orbits. The least 

 distance of Hygeia from the sun exceeds the greatest 

 distance of Flora by more than twenty-five millions of 

 miles. In order to render an intersection of these orbits 

 possible, we must suppose a great variation of the ec- 

 centricity. But the change of the eccentricity of the 

 planetary orbits is exceedingly slow, and the present rate 

 of increase of the eccentricity of Yesta must be con- 

 tinued twenty-seven thousand years to render the aphelion 

 distance of that planet equal to the perihelion distance 

 of Hygeia. Moreover, the eccentricity of the orbit of 

 Yesta is now increasing, which implies that in past ages 

 the interval between Yesta and Hygeia must have been 

 greater than it is at present; whence the conclusion 

 seems irresistible, that the orbits of Yesta and Hygeia 

 can not have intersected for several myriads of years. 

 When the secular variations of the elements of each of 

 the asteroids have been computed, astronomers will be 

 able to . assign a limit of time beyond which the inter- 



