114 THE MOTION OF A PERFECT LIQUID. 



netic and electric forces appear to act. We are possibh' further from 

 realizing- the actual nature of these forces than from a correct concep- 

 tion of the real nature of a liquid. We have long agreed to abandon 

 the old ideas of the electrical and mag'netic fluids flowing along these 

 lines, and to substitute instead the idea that these lines represent 

 merely the directions in which the forces act. Now we can easily see 

 that this conception is quite a reasonable one, for in the case of the 

 model it is not necessary to have the row of balls actually moving in 

 order that the effect ma}' be transmitted along the different lines they 

 occupy. If I attempt to raise the plate upon which they rest, the 

 pressure is instantly transmitted through the whole row to the top ball 

 along each line, whatever curve the line may take. In the same way, 

 3^ou Avill remember that it was not necessary to have the color bands 

 actually in motion, for, though apparently free to move in any direc- 

 tion, they retain their form for a considerable time, and the path along 

 which thev would influence each other as soon as the tap is opened 

 would ])e along those lines in which the liquid was flowing Ix^fore it 

 was brought to rest. Hence it is possible, with some suitable means, 

 to cause a viscous licjuid to reproduce exactly the lines of magnetic 

 and electrical induction. In the case of magnetism and electricity it 

 is, of course, possible, b}^ means of a small magnetic needle or a gal- 

 vanometer, by exploring the whole surface through which magnetic 

 induction or electrical flow is acting, to plot the lines of force for 

 innumerable cases, where we can work in air or on the surface of the 

 solid conductor. 



But in this building it seems natural to take as an example the case 

 lirst used by the great man to whom the conception of lines of mag- 

 netic force is due, for the first reference 1 have been able to find to 

 such lines is in one of Faraday's earliest papers on the indication of 

 electric currents (Experimental Researches in Electricity, Vol. I, p. 82), 

 in which he says: 



"By magnetic curves I mean the lines of magnetic forces, however 

 modified by the juxtaposition of poles, which would be depicted by 

 iron filings, or those to which a very small magnetic needle would form 

 a tangent." 



You are all familiar with the way in which iron filings set themselves 

 when shaken over the north and south poles of a magnet. The mag- 

 netic lines are then nearly, but not quite, circular curves between the 

 two poles. Now, the mathematics of the subject tell us that if the 

 poles could be regarded as points, the lines of force between them 

 would be perfect circles. 



You are now looking at the color bands, the edges, or indeed an}^ 

 portion of which represent lines obtained by admitting colored liquid 

 from a series of small holes round a central small orifice which admits 

 clear liquid and allows them to escape through another small orifice 



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