534 THE PRINCIPLES OF SCIENCE. [CHAP 



open to the affinity of sulphur. The uniform strength and 

 direction 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 a heavier body to displace a lighter 

 one, while the direction from east to west was explained 

 as a result of the earth's rotation. Whatever body in the 

 northern hemisphere changes its latitude, whether it be a 

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

 the right hand. Dove's law of the winds is 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, or hypothesis, 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 safe and extensive 

 inference. Each recorded result informs us exactly what 

 will be experienced again in the same circumstances, 

 but has no bearing upon what will happen in other cir- 

 cumstances. 



Overlooked Results of Tlwory. 



We must by no means suppose that, when a scientific 

 truth is in our possession, all its consequences will be 

 foreseen. Deduction is 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 deduction will lead the reasoner to every result of a law 

 or combination of laws. Whatever road a traveller takes, 

 he is sure to arrive somewhere, but unless he proceeds in 

 a systematic manner, it is unlikely that he will reach 

 every place to which a network of roads will conduct him. 



In like manner there are many phenomena which were 

 virtually within the reach of philosophers by inference from 

 their previous knowledge, but were never discovered .until 

 accident or systematic empirical observation disclosed their 

 existence, 



