xxiv.] EMPIRICAL KNOWLEDGE, EXPLANATION, &c. 547 



of inversion. As Professor Tyndall has remarked, Faraday 

 had a profound belief in the reciprocal relations of the 

 physical forces. The great starting-point of his researches, 

 the discovery of electro-magnetism, was clearly an inversion, 

 Oersted and Ampere had proved that with an electric cur- 

 rent and a magnet in a particular position as antecedents, 

 motion is the consequent. If then a magnet, a wire and 

 motion be the antecedents, an opposite electric current will 

 be the consequent. It would be an endless task to trace 

 out the results of this fertile relationship. Another part of 

 Faraday's researches was occupied in ascertaining the direct 

 and inverse relations of magnetic and diainagnetic, amor- 

 phous and crystalline substances in various circumstances. 

 In all other relations of electricity the principle of inver- 

 sion holds. The voltameter or the electro-plating cell is 

 the inverse of the galvanic battery. As heat applied to a 

 junction of antimony and bismuth bars produces electricity, 

 it follows that an electric current passed through such 

 a junction will produce cold. But it is now sufficiently ap- 

 parent that inversion of cause and effect is a most fertile 

 means of discovery and prediction. 



Facts known only by Theory. 



Of the four classes of facts enumerated in p. 525 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 pheno- 

 menon which people can verify with their own eyes. But 

 it is surely a matter for greater wonder that a physicist 

 describes and measures phenomena which eye cannot see, 

 nor sense of any kind detect. 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 instru- 

 ments as at present constructed. But there is a class of 

 yet more remarkable cases, in which 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 



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