A DYNAMICAL THEORY OF THE ELECTROMAGNETIC FIELD. 527 



kind is that of M. W. Weber*, who has made the same theory include elec- 

 trostatic and electromagnetic phenomena. 



In doing so, however, he has found it necessary to assume that the force 

 between two electric particles depends on their relative velocity, as well as on 

 their distance. 



This theory, as developed by MM. W. Weber and C. Neumann t, is ex- 

 ceedingly ingenious, and wonderfully comprehensive in its application to the 

 phenomena of statical electricity, electromagnetic attractions, induction of currents 

 and diamagnetic phenomena ; and it comes to us with the more authority, as 

 it has served to guide the speculations of one who has made so great an 

 advance in the practical part of electric science, both by introducing a consistent 

 system of units in electrical measurement, and by actually determining electrical 

 quantities with an accuracy hitherto unknown. 



(2) The mechanical difficulties, however, which are involved in the assump- 

 tion of particles acting at a distance with forces which depend on their velocities 

 are such as to prevent me from considering this theory as an ultimate one, 

 though it may have been, and may yet be useful in leading to the coordina- 

 tion of phenomena. 



I have therefore preferred to seek an explanation of the fact in another 

 direction, by supposing them to be produced by actions which go on in the 

 surrounding medium as well as in the excited bodies, and endeavouring to 

 explain the action between distant bodies without assuming the existence of 

 forces capable of acting directly at sensible distances. 



(3) The theory I propose may therefore be called a theory of the Electro- 

 magnetic Field, because it has to do with the space in the neighbourhood of 

 the electric or magnetic bodies, and it may be called a Dynamical Theory, 

 because it assumes that in that space there is matter in motion, by which 

 the observed electromagnetic phenomena are produced. 



(4) The electromagnetic field is that part of space which contains and 

 surrounds bodies in electric or magnetic conditions. 



* " Electrodynamische Maassbestimmnngen." Leipzic Trans. Vol. I. 1849, and Taylor's Scientific 

 Memoirs, Vol. v. art. xiv. 



t Explicare tenlatur quomodo fiat ut lucis planum polarizationis per vires electricas vel magneticas 

 declinetur. Halis Saxonum, 1858. 



