IN THE HISTORY OF SCIENCE. [145] 



they will arrange themselves in certain curves, each proceeding from the N. to the S. pole 

 of the magnet, like the meridians in a map of the globe. It is easily shewn, on the 

 supposition of magnetic attraction and repulsion, that these magnetic curves, as they are 

 termed, are each a curve whose tangent at every point is the direction of a small line 

 or particle, as determined by the attraction and repulsion of the two poles. But if we 

 suppose a magnetic vortex constantly to flow out of one pole and into the other, in streams 

 which follow such curves, it is evident that such a vortex, being supposed to exercise 

 material pressure and impulse, would arrange the iron filings in corresponding streams, 

 and would thus produce the phenomenon which I have described. And the hypothesis of 

 central torrents of Bernoulli or Le Sage which I have referred to, would, in its applica- 

 tion to magnets, really become this hypothesis of a magnetic vortex, if we further suppose 

 that the matter of the torrents which proceed to one pole and from the other, mingles 

 its streams, so as at each point to produce a stream in the resulting direction. Of 

 course we shall have to suppose two sets of magnetic torrents ; — a boreal torrent, proceeding 

 to the north pole, and from the south pole of a magnet ; and an austral torrent proceeding 

 to the south and from the north pole : — and with these suppositions, we make a transition 

 from the hypothesis of attraction and repulsion, to the Cartesian hypothesis of vortices, 

 or at least, torrents, which determine bodies to their magnetic positions by impulse. 



Of course it is to be expected that, in this as in the other case, when we follow the hypo- 

 thesis of impulse into detail, it will need to be loaded with so many subsidiary hypotheses, 

 in order to accommodate it to the phenomena, that it will no longer seem tenable. But 

 the plausibility of the hypothesis in its first application cannot be denied : — for, it may 

 be observed, the two opposite streams would counteract each other so as to produce no 

 local motion, only direction. And this case may put us on our guard against other 

 suggestions of forces acting in curve lines, which may at first sight appear to be 

 discerned in magnetic and electric phenomena. Probably such curve lines will all be 

 found to be only resulting lines, arising from the direct action and combination of 

 elementary attraction and repulsion. 



5. There is another case in which it would not be difficult to devise a mode of transition 

 from one to the other of two rival theories ; namely, in the case of the emission theory 

 and the undulation theory of Light. Indeed several steps of such a transition have already 

 appeared in the history of optical speculation ; and the conclusive objection to the emission 

 theory of light, as to the Cartesian theory of vortices, is, that no amount of additional hypo- 

 theses will reconcile it to the phenomena. Its defenders had to go on adding one piece 

 of machinery after another, as new classes of facts came into view, till it became more complex 

 and unmechanical than the theory of epicycles and eccentrics at its worst period. Otherwise, 

 as I have said, there was nothing to prevent the emission theory from migrating into the 

 undulatory theory, and as the theory of vortices did into the theory of attraction. For 

 the emissionists allow that rays may interfere ; and that these interferences may be modified 

 by alternate Jits in the rays ; now these fits are already a kind of undulation. Then again the 

 phenomena of polarized light shew that the fits or undulations must have a transverse 

 character: and there is no reason why emitted rays should not be subject to Jits of transverse 

 modification as well as to any other fits. In short, we may add to the emitted rays of the 

 one theory, all the properties which belong to the undulations of the other, and thus 

 Vol. IX. Part II. 43 



