CONTRIBUTIONS TO SCIENCE. 



551 



magnetism, which is based on the assumption that the action 



between two quantities of electricity is direct action at a 



distance, and depends not only 



on the distance between the 



charges but upon their relative 



motion. 



From the assumption of both 

 these papers we may draw the 

 conclusions first, that action 

 and reaction are not always equal 

 and opposite ; and second, that 

 apparatus may be constructed to 

 generate any amount of work 

 from its own resources. 



I think that these remark- 

 able deductions from the latest 

 developments of Weber and 

 Neumann's theory can only be 

 avoided by recognising the 

 action of a medium in electrical 

 phenomena. 



While at the Cavendish 

 Laboratory Maxwell construe- c 

 ted a mechanical model which 

 illustrates in a very beautiful 

 manner the principal pheno- 

 mena of induced 'currents. As 

 a piece of mechanism it is 

 simply a differential train, such 

 as is often employed as a 

 dynamometer for measuring 

 the power absorbed by a 

 machine. The apparatus is 

 sketched in Fig. 13. The 

 grooved wheel P is keyed to 

 the same shaft as the bevel Fig 13i 



wheel A, which therefore turns 

 with it, and the rotation of this piece represents the primary 



