622 THE PRINCIPLES OF SCIENCE. [CHAP. 



most beautiful examples of crucial experiments and ob- 

 servations, instances are not wanting in other branches of 

 science. Copernicus asserted, in opposition to the ancient 

 Ptolemaic theory, that the earth moved round the sun, and 

 he predicted that if ever the sense of sight could be 

 rendered sufficiently acute and powerful, we should see 

 phases in Mercury and Venus. Galileo with his telescope 

 was able, in 1610 to verify the prediction as regards Venus, 

 and subsequent observations of Mercury led to a like con- 

 clusion. The discovery of the aberration of light added a 

 new proof, still further strengthened by the more recent 

 determination of the parallax of fixed stars. Hooke pro- 

 posed to prove the existence of the earth's diurnal motion 

 by observing the deviation of a falling body, an experi- 

 ment successfully accomplished by Bcnzenberg ; and 

 Foucault's pendulum has since furnished an additional 

 indication of the same motion, which is indeed also 

 apparent in the trade winds. All these are crucial facts in 

 favour of the Coperuican theory. 



Descriptive Hypotheses. 



There are hypotheses which we may call descriptive 

 hypotheses, and which serve for little else than to furnish 

 convenient names. When a phenomenon is of an unusual 

 kind, we cannot even speak of it without using some 

 analogy. Every word implies some resemblance between 

 the thing to which it is applied, and some other thing, 

 which fixes the meaning of the word. If we are to speak 

 of what constitutes electricity, we must search for the 

 nearest analogy, and as electricity is characterised by the 

 rapidity and facility of its movements, the notion of a fluid 

 of a very subtle character presents itself as appropriate. 

 There is the single-fluid and the double-fluid theory of 

 electricity, and a great deal of discussion has been uselessly 

 spent upon them. The fact is, that if these theories be 

 understood as more than convenient modes of describing 

 the phenomena, they are altogether invalid. The analogy 

 extends only to the rapidity of motion, or rather the fact 

 that a phenomenon occurs successively at different points 

 of the body. The so-called electric fluid adds nothing to 

 the weight of the conductor, and to suppose that it really 



