168 THE PRINCIPLES OF SCIENCE. 



The uniform direction and strength of the trade-winds 

 were long familiar to mariners, before they were explained 

 by Halley on hydrostatical principles. The winds were 

 found to arise from the action of gravity, which causes 

 any heavy body to displace a lighter one, while the direc- 

 tion from east to west was also explained as a necessary 

 result of the earth's rotation. Whatever body moves in 

 the northern hemisphere from north to south, whether it 

 be a bird, or a railway train, or a body of air, must tend 

 towards the right hand, or west. Dove's law of the 

 winds is to the effect that the winds tend to veer in 

 the northern hemisphere in the direction N.E.S.W., and 

 in the southern hemisphere in the direction N.W.S.E. 

 This tendency was shown by him to be the necessary effect 

 of the same conditions which apply to the trade-winds. 

 Whenever, then, any fact is connected by resemblance, law, 

 theory, hypothesis, or any other process of reasoning, with 

 other facts, it is explained. 



Although the great mass of recorded facts must be 

 empirical, and awaiting explanation, such knowledge is 

 of minor value, because it does not admit of extensive and 

 safe inference. Each recorded result informs us exactly 

 what will be experienced again in the same circum- 

 stances, but has no bearing upon what will happen in 

 other circumstances. 



Overlooked Results of Theory. 



We must by no means suppose that, even when a 

 scientific truth is firmly in our possession, all its con- 

 sequences will be foreseen. Deduction is, as I have 

 frequently remarked, certain and infallible, in the sense 

 that each step in deductive reasoning will lead us to some 

 result, as certain as the law itself. But it does not follow 

 that every mode of deducing a fact from a law, or a 



