THE MOTION OF A PERFECT LIQUID. 115 



(called, respectively, in hydroniechanics tx "source and sink), and I leave 

 it to you to judge how far these curves deviate from the ideal form. 



My assistant is now allowing the color to flow, first steadily, and 

 then in a series of impulses, and the latter gives up the conception of 

 waves or impulses of magnetic force, though of course the magnetic 

 transmission force would be instantaneous. Regarded as a liquid, it 

 is here again clear how absolutely the truth of our views concern- 

 ing the slower movement in the wider portion is verified by this 

 experiment. 



A last experiment shows the streams admitted, not from a source, 

 but from a row of orifices in what corresponds to the slowest moving 

 portion of the flow. The result is that the color bands are much nar- 

 rower, and although the circular forms of the curves are, as in the 

 previous experiment, preserved, the lines are so fine at the point of 

 exit, which, as before, corresponds to the South Pole, as to realh'' 

 approximate to ideal stream lines. 



The same method enables us to trace the lines of force through solid 

 conductors, for as long as we confine ourselves to two dimensions of 

 space we may have ^flat conductors of any shape whatever. But it 

 does something more, for by making the film rather deeper in some 

 places than others more particles arrange themselves there, and the 

 lines of flow will naturally tend in the direction of the deeper portion. 

 This will give the stream lines identically the same shape as the mag- 

 netic or electrical curves which encounter in their paths a body of less 

 resistance — for instance, a paramagnetic body. 



If, on the other hand, at these points the film is made rather thinner, 

 less particles will be able to dispose of themselves in the shallow por- 

 tion of the film, and hence the lines of flow will be pushed awa}^ from 

 this portion, giving us exactly the same forms as magnetic lines of 

 force in a magnetic field in proximity to a diamagnetic body. 



Here, again, mathematical methods have enabled lines of actual flow 

 to l)e predicted, and 3'ou may compare the actual flow for the case of 

 a cylindrical paramagnetic body which was worked out some years ago. 



You will doubtless not be inclined to question the practical value of 

 stream lines in the subject which we have just been considering, 

 because, unlike the flow of an actual liquid, magnetic lines of force 

 can never be themselves seen, and Ijecause there is no doubt as to the 

 correspondence of the directions to the lines of a perfect liquid. It 

 was the conception of these lines in the mind of Faraday, and more 

 particularly their being cut by a moving wire, that enabled him to 

 realize the nature of the subject more clearl}^ than any other man at 

 the time, and to do much toward the rapid development of electrical 

 science and its practical applications. 



When we come to consider the relation of the study of the motion 

 of a perfect liquid with hydromechanics and naval architecture, it must 



