130 Mr. O. Heaviside on Electro mag netic Waves, and the 



secure for all the particles of a gas/' Mutual collisions secure 

 the continual change of the individual particles constituting a 

 particular class, and the diminution of a, and that is all the 

 influence they have in the case under consideration. 



I find, however, that in one respect I misunderstood Pro- 

 fessor Tait. I said that on his hypothesis a, the translation- 

 velocity of the class r in the moving gas. would be constant. 

 But that would not be the effect of mass motion. If we give 

 to a gas, previously at rest, mass motion v, the number of 

 molecules in the gas so moving whose velocities are between 

 v and v + dv, and make with the direction u angles between i/r 

 and yjr + dTJr, is 



Oe-fc-V dr(l + Uuv cos yfr) s }R±J l ± . 



and therefore the mean translation-velocity of the class /• is, 

 not u, but ^hv^u. In order to represent Professor Tail's 

 hypothesis I should make in my notation a=§/u ,2 «. But it 

 would not satisfy the condition for steady motion. 



XV. On Electromagnetic Waves, especially in relation to the 

 Vorticity of the Impressed Forces ; and the Forced Vibra- 

 tions of Electromagnetic Systems. By Oliver Heaviside*. 



1. NUMMARY of Electromagnetic Connexions. — To avoid 

 indistinctness, I start with a short summary of Jlax- 

 well's scheme, so far as its essentials are concerned, in the 

 form given by me in January 1885 f. 



Two forces, electric and magnetic, E and H, connected 

 with the three fluxes, — electric displacement D, conduction- 

 current C, and magnetic induction B ; thus 



B = ^H, C = A;E, D = (c/4tt)E. ... (1) 

 Two currents, electric and magnetic, T and G, each of 

 which is proportional to the curl or vorticity of the other force 

 not counting impressed ; thus, 



curl (H— h) = 47rr, (2) 



curl (e — E) =47rG ; (3) 



where e and h are the impressed parts of E and H. These 

 currents are also directly connected with the correspondino- 

 forces through 



F = C + b, G = B/4tt (4) 



* Communicated by the Author. 



t See the opening' sections of "Electromagnetic Induction and its 

 Propagation/' Electrician, Jan. 3, 1685, and after. 



