364 THE ASTEROIDS BETWEEN MARS AND JUPITER. 



the orbits of Mercury and Mars. Even the least distance of Hilda ex- 

 ceeds the greatest of Harmonia by one hundred and twenty millions of 

 miles. It is, therefore, no more probable that the asteroids have all been 

 produced by the disruption of a single planet, than that Mercury, Venus, 

 the Earth, and Mars originated in a similar manner. The relative dis- 

 tances of Hilda and Flora, together with those of the four planets interior 

 to the latter, are represented in the following figure. 



1. Orbit of Hilda. 



2. Orbit of Flora. 



3. Orbit of Mars. 



4. Orbit of Earth. 



5. Orbit of Venus. 



6. Orbit of Mercury. 



4. The small mass of the asteroids. — In taking a general view of the solar 

 system, we cannot fail to be struck by the remarkable fact that Jupiter, 

 whose mass is much greater than that of all the other planets united, 

 should be immediately succeeded by a region so nearly destitute of 

 matter as the zone of asteroids. Leverrier, without attempting to deter- 

 mine the mass of the minor planets, infers from the motion of Mars's 

 perihelion that it is certainly less than one-fourth of the earth's mass, 

 or ^ Jq^ of Jupiter's. Prof. Stephen Alexander estimates the mass at 

 5i 3 i¥T4 ^^ ^^^ mass of the sun.* In other words, its ratio to the mass 

 of Jupiter is that of 1 to 5180. We find, also, analogous facts in the 

 secondary systems. Jupiter's third satellite, the largest of the number, 

 is nearly four times greater than the second. Immediately within the 

 orbit of Titan, the largest satellite of Saturn, occurs a wide hiatus, and 

 the volume of the next interior satellite is to that of Titan in the ratia 

 of 1 to 21. In the Uranian system, the widest interval between adjacent 

 orbits is just within the orbit of the bright satellite Titania. No other 

 secondary planet of our system contains so much matter in proportion 

 to its primary as our own moon. Did this powerful mass prevent the 

 formation or permanent existence of interior satellites ? 



5. The foregoing facts suggest the inquiry. What effect would be 

 produced by a large planet on interior masses abandoned by a central 

 spheroid ? As the phenomena in all instances would be of the same 

 nature, we will consider a single case — that of Jupiter and the asteroids. 



The powerful mass of the exterior body would produce great pertur- 

 bations of the rings or gaseous masses thrown off from the equator of 

 the solar spheroid. The disturbed orbits, in some cases, would thus at- 

 tain considerable eccentricity, so that the matter moving in them would, 

 in perihelio, be brought in contact with the equatorial parts of the cen- 

 tral nebula, and thus become reunited with it. The extreme rarity of 

 the zone between Mars and Jupiter, regarded as a single ring, is thus 

 accounted for in accordance with known dynamical laws. 



* Smithsoniau Contributions to Knowledge, No. 280, p. 33. 



