226 FACTS AND THEORIES. 



that the world was a true sphere with diameters all 

 equal. But was it so ? 



Scientists differed. All agreed that if it was a sphere, 

 degrees of latitude everywhere would be of the same 

 length, and, if it was an oblate spheroid, as seemed to 

 be indicated by the shortening of the pendulum near the 

 equator, and by the necessary effect of centrifugal ac- 

 tion, a degree of latitude measured to the far north, 

 ought to be longer than one measured near the equator. 

 The question was put, two such degrees were measured 

 with all the care possible, and the polar degree was 

 found to be the longer. The earth therefore, is a sphere 

 flattened at the poles. 



More complex theories present greater difficulty in 

 properly framing the questions. The principle, however, 

 and the method are substantially the same. 



A third office of theories is to serve as hypothetical 

 foundations on which to build logical structures, which 

 have sometimes led to discoveries of great importance. 

 A theory as to the perturbations of Uranus led Leverrier 

 to the discovery of Neptune. 



Lastly, theories serve as a scaffolding on which to 

 stand while the work is going on, and, like other scaf- 

 folding, are destined to be removed when the structure 

 is completed. 



What I have said, will I hope, convince those who are 

 not scientists — scientists know it now — that theories are 

 in their nature only tentative, temporary expedients, 

 helps to be used until replaced by something better. In 

 their place they are very important, so much so that 

 without their assistance very little progress would be 

 possible. But woe to the man who builds on them, 

 thinking he has bed-rock under him. Some morn- 

 ing he will wake up, and find his foundation gone, and 

 his superstructure, reared with so much logic, vanished. 



Theories may become dangerous. Not as in the past 



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