BOOK III. 



METHODS OF MEASUREMENT. 



CHAPTER XIII. 



THE EXACT MEASUREMENT OF PHENOMENA. 



As physical science advances, -it becomes more and 

 more accurately quantitative. Questions of simple logical 

 fact after a time resolve themselves into questions of 

 degree, time, distance, or weight. Forces hardly suspected 

 to exist by one generation, are clearly recognised by the 

 next, and precisely measured by the third generation. 

 But one condition of this rapid advance is the invention 

 of suitable instruments of measurement. We need what 

 Francis Bacon called Instantice citantes, or evocantes, 

 methods of rendering minute phenomena perceptible to 

 the senses ; and we also require Instantice radii or curri- 

 culi, that is measuring instruments. Accordingly, the 

 introduction of a new instrument often forms an epoch in 

 the history of science. As Davy said, " Nothing tends so 

 much to the advancement of knowledge as the application 

 of a new instrument. The native intellectual powers of 

 men in different times are not so much the causes of the 

 different success of their labours, as the peculiar nature 

 of the means and artificial resources in their possession." 

 In the absence indeed of advanced theory and analyti- 



