442 Scientific Proceedings, Royal Dublin Society. 



attention to tHe purely analogical nature of the description of 

 electric currents as currents, and electric displacements as displace- 

 ments. The analogy between electric currents and currents of an 

 incompressible fluid is so complete and so easily drawn, that there 

 is serious risk of our being as much hampered in future advances 

 by our imagining this analogy to be a likeness as we have been in 

 the past by the theories of action at a distance, which still control 

 the text-books and the ordinary methods of speaking of electrical 

 phenomena to such an extent that most people find it almost im- 

 possible to think of electric and magnetic phenomena in any other 

 light. For fear of our being similarly hampered by the words 

 " electric displacement " and " electric current," I think it worth 

 while pointing out that just as these words presuppose an analogy 

 between electric quantity and volume of liquid, and electric poten- 

 tial and pressure of liquid, similarly it is quite possible to draw 

 another analogy, namely between electric quantity and entropy, 

 and electric potential and temperature, so that, with these two dif- 

 ferent analogies in view, it may be the less likely that the student 

 should look upon either as anything more than an analogy, until 

 further investigation shows that there is either a real likeness to 

 one or the other, or that the electric phenomenon is sui generis, 

 and unlike either, which latter is in my opinion the more probable 

 supposition. 



