298 THE PRINCIPLES OF SCIENCE. 



between the Copernican, and Ptolemaic systems, it was 

 analogy which furnished the most satisfactory arguments. 

 Galileo discovered, by the use of his new telescope, the 

 four small satellites which circulate round Jupiter, and 

 make a miniature planetary world. These four Medi- 

 cean Stars, as they were called, were plainly seen to re- 

 volve round Jupiter in various periods, but approximately 

 in one plane, and astronomers irresistibly inferred that 

 what might happen on the smaller scale might also be 

 found true of the greater planetary system. This dis- 

 covery gave the holding turn, as Sir John Herschel has 

 expressed it, to the opinions of mankind. Even Francis 

 Bacon, who had, in a manner little to the credit of his 

 scientific sagacity, previously opposed the Copernican 

 views, now became partially convinced, saying ' We affirm 

 the solisequium of Venus and Mercury ; since it has been 

 found by Galileo that Jupiter also has attendants.' Nor 

 did Huyghens think it superfluous to adopt the analogy 

 as a valid argument k . Even in an advanced stage of the 

 science of physical astronomy, the Jovian system has not 

 lost its analogical interest ; for the mutual perturbations 

 of the four satellites pass through all their phases within 

 a few centuries, and thus enable us to verify in a minia- 

 ture case the principles of stability, which Laplace has 

 established for the great planetary system. Oscillations 

 or disturbances which in the motions of the planets appear 

 to be secular, because their periods extend over millions 

 of years, can be watched, in the case of Jupiter's satellites, 

 through complete revolutions within the historical periods 

 of astronomy 1 . 



In obtaining a knowledge of the stellar universe we 

 must depend much upon somewhat precarious analogies. 

 We must start with the opinion, entertained by Bruno as 



k ' Cosmotheoros' (1699), p. 16. 



1 Laplace, 'System of the World,' vol. ii. p. 316. 



