CONTRIBUTIONS TO SCIENCE. 519 



tubes above described is possible. Faraday pointed out that 

 there is not only a tension exerted along each line of force, 

 but that the several lines exert a repulsion upon one another, 

 and Maxwell showed that a tension along the lines of force, 

 accompanied by an equal pressure in every direction at right 

 angles to these lines, is consistent with the equilibrium of 

 the medium. Taking an illustration from the flow of water 

 in a river, Maxwell pointed out that the stream lines or paths 

 along which particles of water flow, are analogous to lines of 

 electric force, the velocity of the water being analogous to 

 the intensity of the force. If the river be supposed to be 

 divided into tubes, the boundaries of which are lines of flow, 

 and if these tubes be so drawn that unit volume of water 

 passes across a particular section of each tube in a second, 

 then, if the flow be steady, unit volume of water will flow 

 across every section of each tube in a second, since no water 

 enters or leaves the tube except at its ends. Such tubes 

 may be called unit tubes of flow, and if no tributaries enter 

 the river there will be the same number of unit tubes crossing 

 each section of the river. Where the bed widens the section 

 of each tube increases, being always inversely proportional 

 to the velocity of the water, and hence the number of unit 

 tubes of flow which cut any unit of area in a cross section of 

 the river will be proportional to the velocity of the water in 

 the neighbourhood. Such a system of tubes, therefore, will 

 represent both the direction of motion and velocity of the 

 water at every point, and will exactly correspond, mutatis 

 mutandis, with a system of unit tubes of electric force. 



The following letter was addressed to Maxwell by 

 Faraday on receiving a copy of the paper on "Lines of 

 Force:" 



Albemarle Street, W., 25th March 1857. 



MY DEAR SIR I received your paper, and thank you very 

 much for it. I do not say I venture to thank you for what you 

 have said about " Lines of Force," because I know you have done 

 it for the interests of philosophical truth ; but you must suppose 

 it is work grateful to me, and gives me much encouragement to 

 think on. I was at first almost frightened when I saw such 

 mathematical force made to bear upon the subject, and then 



