Xcii REPORT — 1871. 



lowing the same law of variation with distance urges the Moon towards the 

 Earth. Then first, we may suppose, came to him the idea of the universality of 

 gravitation ; butwhen he attempted to compare the magnitude of the force on the 

 Moon with the magnitude of the force of gravitation of a heavy body of equal 

 mass at the earth's surface, he did not find the agreement which the law he 

 was discovering required. Not for years after would he publish his discovery 

 as made. It is recounted that, being present at a meeting of the Royal Society, 

 he heard a paper read, describiug geodesic measurement by Picard which 

 led to a serious correction of the previously accepted estimate of the Earth's 

 radius. This was what Newton required. He went home with the result, 

 and commenced liis calculations, but felt so much agitated that he handed 

 over the arithmetical work to a friend : then (and not when, sitting in a 

 garden, he saw an apple fall) did he ascertain that gravitation keeps the Moon 

 in her orbit. 



Faraday's discovery of specific inductive capacity, which inaugurated the 

 new philosophy, tending to discard action at a distance, was the result of 

 minute and accurate measurement of electric forces. 



Joule's discovery of thermo-dynamic law through the regions of electro- 

 chemistry, electro-magnetism, and elasticity of gases was based on a delicacy 

 of thermometry which seemed simply impossible to some of the most dis- 

 tinguished chemists of the day. 



Andrews' discovery of the continuity between the gaseous and liquid states 

 was worked out by many years of laborious and minute measurement of phe- 

 nomena scarcely sensible to the naked eye. 



Great service has been done to science by the British Association in pro- 

 moting accurate measurement in various subjects. The origin of exact 

 science in terrestrial magnetism is traceable to Gauss' invention of methods 

 of finding the magnetic intensity in absolute measure. I have spoken of 

 the great work done by the British Association in carrying out the ap- 

 plication of this invention in all parts of the world. Gauss' colleague in 

 the German Magnetic Union, AVebcr, extended the practice of absolute 

 measurement to electric currents, the resistance of an electric conductor, 

 and the electromotive force of a galvanic element. He showed the rela- 

 tion between electrostatic and electromagnetic units for absolute mea- 

 surement, and made the beautiful discovery that resistance, in absolute elec- 

 tromagnetic measure, and the reciprocal of resistance, or, as we call it, " con- 

 ducting power," in electrostatic measure, are each of them a velocity. He 

 made an elaborate and difficult series of experiments to measure the velocity 

 which is equal to the conducting power, in electrostatic measure, and at the 

 same time to the resistance in electromagnetic measure, in one and the same 

 conductor. Maxwell, in making the first advance along a road of which 

 Faraday was the pioneer, discovered that this velocity is physically related to 

 the velocity of light, and that, on a certain hypothesis regarding the elastic 

 medium concerned, it may be exactly equal to the velocity of light. Weber's 

 measurement verifies approximately this equality, and stands in science 

 monumentmn cere perennius, celebrated as having suggested this most grand 

 theory, and as having afforded the first quantitative test of the recondite 

 properties of matter on which the relations between electricity and light 

 depend. A remeasurcment of Weber's critical velocity on a new plan by Max- 

 well himself, and the important correction of the velocity of light by Fou- 

 cault's laboratory experiments, verified by astronomical observation, seem to 

 show a still closer agreement. The most accurate possible determination of 

 Weber's critical velocity is just now a primary object of the Association's 



