ADVANCE OF ASTRONOMY. 183 



In illustrating the century^s confirmations and ex- 

 tensions of the gravitational theory, account should 

 be taken of re-estimates of the sun^s distance, re- 

 investigations of the movements of the moon and the 

 planets, further elaboration of the theory of the 

 tides, and so on. We have confined ourselves to a 

 brief notice of the discovery of the minor planets, 

 the discovery of N^eptune, and the study of comets. 



Discovery of the Minor Planets. — Kant had sug- 

 gested that the zone in which a planet moves might 

 be regarded as the empty area from which its ma- 

 terials had been derived, and that some definite re- 

 lation should therefore be found between the masses 

 of the planets and the intervals between them. In 

 .1772 Titius pointed out that the distances from 

 the sun of the six planets then known might be 

 represented by a certain numerical series, except that 

 there was nothing to correspond to the term succeed- 

 ing the one which corresponds to the orbit of Mars. 

 Johann Elert Bode, astronomer in Berlin, filled the 

 gap with a hypothetical planet, and the search for 

 it began. In 1801 Piazzi discovered Ceres, and with 

 the help of Gauss's mathematical genius (used to pre- 

 dict where the planet should be at certain dates) von 

 Zach and Olbers were soon able to confirm Piazzi. 

 In spite of HegeFs protest that the number of planets 

 could not exceed the sacred number seven, a second 

 (Pallas) was soon discovered (1802) by Olbers, 

 and in 1807 four were known. Three of these " as- 

 teroids,'' as Sir W. Herschel called them, corre- 

 sponded approximately with the requirements of the 

 series indicated by Titius and usually referred to as 

 " Bode's LaAV," and the idea commended itself that 

 these bodies were the remains of an exploded planet. 



As we now know, neither Bode's Law nor the no- 



