ZONE OF PLANETS BETWEEN MARS AND JUPITER. 91 



has arrived at the conclusion that the sum total of the 

 matter constituting the small planets situated between 

 the mean distances 2*20 and 3*16 (including undiscovered 

 as well as known asteroids), can not exceed about one 

 fourth of the mass of the earth. 



2. The asteroids in their position occupy a zone en- 

 tirely distinct from the other planets of the solar system. 

 Between the orbits of Jupiter and Saturn between 

 Saturn and Uranus is an immense interval, furnishing 

 space enough for a host of little bodies to circulate around 

 the sun ; but in not a solitary instance has any such body 

 been found, except between Mars and Jupiter. Some 

 may attempt to account for this circumstance by saying 

 that astronomers have long been watching exclusively 

 this portion of space, and have left all other regions 

 entirely unexplored. An exploration, conducted upon 

 such a principle, is simply a physical impossibility. If 

 there were a small planet between the Earth and Mars, 

 it would have stood the same chance of detection, in the 

 explorations of the past ten years, as if it were situated 

 between Mars and Jupiter; and, indeed, it would have 

 stood a better chance of detection, inasmuch as it would 

 appear of greater brightness on account of its proximity 

 to us. If there were a small planet circulating between 

 Jupiter and Saturn, it would have stood the same chance 

 of detection as if it had been placed this side of Jupiter, 

 except that it would appear somewhat fainter on account 

 of its increased distance. The fact that we have dis- 

 covered forty small planets between Mars and Jupiter, 



