242 SECTION MECHANICS. 



exception of this minor proposition of Cavendish's syllogism, and the 

 great physical discovery of Faraday of the peculiar inductive quality 

 known as the electrostatic inductive capacity of dielectrics. With these 

 two exceptions the whole theory of electrostatics was completed in 

 the last century. It was left for us to work out the mathematical 

 conclusions from the theory of Cavendish, Coloumb, and Robinson ; 

 and it was not until after the end of the last century that the existence 

 of electro-magnetic force became known. Orsted made the great dis- 

 covery in 1820 of the mutual connection between a magnet and a 

 wire in which an electrical current is flowing, and the remarkable 

 developments which were very speedily given to that discovery by 

 Ampere, led to the foundation of the other great branch of electrical 

 science, and pointed to the subject of electro-magnetic measurement, 

 upon which I must say a word or two now. 



I think the principles of the mathematical theory of the inter- 

 action of wires containing currents mutually between one another, 

 and again their mutual action upon magnets, was fully laid down by 

 Ampere in consequence of Orsted's discovery. The working out of 

 the accurate measurement of currents, and generally of the system of 

 measurement founded on these principles, was founded altogether in 

 Germany. The great work of Gauss and Weber on terrestrial mag- 

 netism belongs strictly to this subject. I believe Gauss first laid 

 down the system of absolute measurement for magnetic force. The 

 definitions and mathematical theory, of Poisson and Coulomb as to 

 magnetic polarity, and the magnetic force founded on it, was worked 

 out practically by Gauss, and made the foundation of the whole system 

 of magnetic measurement followed in our magnetic observatories. 

 This was an immense step in science, and one of great importance, 

 giving, not merely definite measurement, but measurement on a certain 

 absolute scale, which, even if all the instruments by which the mea- 

 surements were made were destroyed, would still give us a perfectly 

 definite result. It was, for the first time in physical science, worked 

 out in consequence of Gauss' foundation of the system for terrestrial 

 magnetism. That, then, is really the beginning of absolute measurement 

 in magnetic science, and for electro-magnetics and electrostatic science. 

 Gauss and Weber carried on the work for terrestrial magnetism 

 together, and Weber carried on by himself, I believe, during Gauss' 



