On Maxwell's Electromagnetic Equations. 29 



The author finds that, by making the leg A about 2 metres 

 long, abundant length can be obtained between D and C to 

 place a small fluid meter, by which means the weight of liquid 

 which passes from D to C can be accurately measured. The 

 instrument works either with a continuous or alternating 

 current. The stopcock B is used to contract the cross section 

 of the U-tube ; without some check a violent oscillation is set 

 up between the two columns of liquid. 



The first experimental instrument constructed by the author 

 consisted of a continuous rectangular-shaped tube, one part of 

 which was heated while the rest of the tube was kept cool. 

 The liquid circulated, and its rate of circulation, when pro- 

 perly interpreted, would have been a measure of what was 

 required ; but it was given up, as it appeared nearly impos- 

 sible to cause the current of liquid to mechanically record its 

 rate of flow with any amount of accuracy. 



Several rather interesting results, which at present are not 

 complete for publication, have cropt out of experiments made 

 with the instrument. Of these, perhaps, the most promising 

 was an illustration of the conversion of work into heat. The 

 meter devised for determining the flow is one by which the 

 weight of the oil is indicated, not the volume. 



IV. The General Solution of Maxwell's Electromagiietic 

 Equations in a Homogeneous Isotropic Medium, especially 

 in regard to the Derivation of special Solutions, and the 

 Formulcefor Plane Waves. By Olivek Heaviside*. 



1- EQUATIONS of the Field.— KMhovigh, from thedifii- 

 culuy of applying them to practical problems, general 

 solutions frequently possess little practical value, yet they may 

 be of sufficient importance to render their investigation desi- 

 rable, and their applications examined as far as may be prac- 

 ticable. The first question here to be answered is this. Given 

 the state of the whole electromagnetic field at a certain moment, 

 m a homogeneous isotropic conducting dielectric medium, 

 to deduce the state at any later time, arising from the initial 

 state alone, without impressed forces. 



The equations of the field are, if j9 stand for djdt, 



curlH = (47r^ + c/))E, (1) 



— curl E=:(47r^ + /i^)H; .... (2) 



the first being Maxwell's well-known equation defining electric 



* Communicated by the Author. 



