398 THE PEINCIPLES OF SCIENCE. [CH. xvn. 



been mentioned that a considerable error was discovered 

 in the determination of the standard metre as the ten- 

 millionth part of the distance from the pole to the 

 equator (p. 314). 



We shall return in Chapter XXV. to the further consi- 

 deration of the methods by which we may as far as possible 

 secure ourselves against permanent and undetected sources 

 of error. In the meantime, having completed the con- 

 sideration of the special methods requisite for treating 

 quantitative phenomena, we must pursue our principal 

 subject, and endeavour to trace out the course by which 

 the physicist, from observation and experiment, collects 

 the materials of knowledge, and then proceeds by hypo- 

 thesis and inverse calculation to induce from them the 

 laws of nature. 



