EMPIRICAL KNOWLEDGE, EXPLANATION, &c. 185 



we might imagine a train of magnetic engines and electro- 

 magnetic machines, which should alternately convert the 

 same energy into motion and electricity. Similarly, by a 

 proper arrangement of bars of antimony and bismuth, the 

 same current of electricity might be converted into heat 

 and reconverted into electricity an indefinite number of 

 times. But, practically speaking, there would be an 

 enormous loss of energy at each conversion, so that the 

 ultimate effect would dwindle down to an inconsiderable 

 fraction of the original amount of energy. 



Facts known only by Theory. 



Of the four classes of facts enumerated in p f 1 5 7 the last 

 remains unconsidered. It includes the unverified pre- 

 dictions of science. Scientific prophecy arrests the atten- 

 tion of the world when it refers to such striking events 

 as an eclipse, the appearance of a great comet, or any 

 other phenomenon which every one can verify with his 

 own eyes. But it is surely a greater matter for wonder 

 that in many cases a physicist may describe and measure 

 phenomena which eye cannot see, nor sense of any kind 

 appreciate. In most cases this arises from the effect being 

 too small in amount to affect our organs of sense, or come 

 within the powers of our instruments as at present con- 

 structed. There is another class of yet more remarkable 

 cases, where a ^phenomenon cannot possibly be observed, 

 and yet we can say what it would be if it were observed. 



In astronomy, systematic aberration is an effect of the 

 sun's proper motion almost certainly known to exist, but 

 which we have no hope of detecting by observation in the 

 present age of the world. As the earth's motion round 

 the sun combined with the motion of light causes the 

 stars to deviate apparently from their true positions to 

 the extent of about 1 8" at the most, so the motion of the 



