1699.] on the Motion of a Perfect Liquid. 61 



But in this "building it soems natural to take as an example the 

 case first used by the great man to whom the conception of lines of 

 magnetic force is dup, for the first reference I have been able to find 

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

 electric currents,* 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 them- 

 selves, when shaken over the North and South poles of a magnet. 

 The magnetic lines are then nearly, but not quite, circular curves 

 between the two poles Now, the mathematics of the subject tells us 

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

 them would be perfect cireles. 



You are now looking (Fig. 17, Plate II.), at the colour bands, the 

 edges — or indeed any portion — of which represent lines obtained by 

 admitting coloured 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 (called respectively in hydromechanics 

 a 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 colour to flow, first steadily and 

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

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

 netic transmission force would be instantaneous. Regarded as a 

 liquid, it is here again clear how absolutely the truth of our views 

 concerning the slower movement in the wider portion is verified by 

 this experiment. 



A last experiment (Fig. 18) 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 colour bands are 

 much narrower, 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 

 really 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 dimen- 

 sions of space, we may have flat conductors of any shape whatever. 

 But it does something more, for by miking the film rather deeper in 

 some places than others, more particles arrange themselves there, 

 and the lines of flnv will naturally tend in the direction of the 

 deeper portion. This will give the stream lines identically the 

 same shape as the magnetic or electrical curves which encounter in 

 their paths a body of less resistance, for instance, a para-magnetic 

 body. 



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



* ' Experimental Researches in Electricity,' vol. i. p. 32 



