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 citantcs, or evocantes, 



methods of rendering minute phenomena perceptible to 



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



eidi, 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 tlie 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 tlie means and artificial resources in tlieir possession." 



In the absence indeed of advanced theory and anaiyti- 



