SECTION II. 



THE ZONE OF PLANETS BETWEEN MARS AND JUPITER, 



SEVENTY-FIVE years since, the only planets known to 

 men of science were the same which were known to 

 the Chaldean shepherds thousands of years ago. Between 

 the orbit of Mars and that of Jupiter, there occurs an 

 interval of no less than 350 millions of miles, in which 

 no planet was known to exist before the commencement 

 of the present century. Nearly three centuries ago, 

 Kepler had pointed out something like a regular pro- 

 gression in the distances of the planets as far as Mars, 

 which was broken in tne case of Jupiter. Having de- 

 spaired of reconciling the actual state of the planetary 

 system with any theory he could form respecting it, he 

 hazarded the conjecture that a planet really existed be- 

 tween the orbits of Mars and Jupiter, and that its small- 

 ness alone prevented it from being visible to astronom- 

 ers. The remarkable passage containing this conjecture 

 is found in his Prodromus, and is as follows : " When this 

 plan, therefore, failed, I tried to reach my aim in another 

 way, of, I must confess, singular boldness. Between 

 Jupiter and Mars I interposed a new planet, and another 

 also between Yenus and Mercury, both which it is pos- 

 sible are not visible on account of their minuteness, and 



