638 THE PRINCIPLES OF SCIENCE. [chap. 



Analogy in Astronomy. 



We shall be much assisted in gaining a true apprecia- 

 tion of the value of analogy in its feebler degrees, by con- 

 sidering how much it has contributed to the progress of 

 astronomical science. Our point of observation is so fixed 

 with regard to the universe, and our means of examining 

 distant bodies are so restricted, that we are necessarily 

 guided by limited and apparently feeble resemblances. In 

 many cases the result has been confirmed by subsequent 

 direct evidence of the most forcible character. 



While the scientific world was divided in opinion 

 between the Copernican and Ptolemaic systems, it was 

 analogy which furnished the most satisfactory argument. 

 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 Medicean 

 Stars, as they were called, were plainly seen to revolve 

 round Jupiter iu 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 discovery gave " the 

 holding turn," as Herschel expressed it, to the opinions of 

 mankind. Even Francis Bacon, who, little to the credit of 

 his scientific sagacity, had previously opposed the Coper- 

 nican views, now became 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. 1 Even in an advanced stage 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 miniature case the principles 

 of stability, which Laplace established for the great plane- 

 tary 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 period of astronomy.^ 



^ Cosmotheoros (1699), p. 16. 



2 Laplace, System of the World, vol. ii. p. 316. 



