578 EEPOET— 1888. 



filaments would atut on this. These would break off, and rearrange themselves so 

 that some of the filaments would end on it and an equal number start from it. 



Call those ends of the filaments positive at which the rotation, looked aXfrom 

 the surface on which they abut, is right-handed ; and those ends negative at which 

 the rotation is left-handed. The two ends of any filament will then always be 

 of opposite name. 



The analogy with electrical action is then close. Two bodies, A B, brought in 

 contact touch in innumerable points, owing possibly to the different states of 

 motion of the ether at their surfaces ; a state of equilibrium is set up, consisting of 

 a kind of sifting of vortex motion already subsisting, so that on the wliole more 

 lines go from A to B than vice versa. This is E.M.F. of contact. Separate the 

 bodies ; one has a number of positive ends of hollow filaments, and the other an 

 equal number of negative ends. The bodies are equally and oppositely clectrijied. 

 The forces in the hydrodynamical case and in its electrical analogue follow the 

 same laws. 



Bring into the field another surface C ; the filaments rearrange themselves as 

 mentioned above, the negative and positive ends abutting on C being equal. This 

 is induced electricity. 



If the surface C consist of a large number of small unconnected surfaces, the 

 filaments will arrange themselves in some manner depending on the sizes and 

 distances of the unconnected parts, the field will be disturbed in a similar way to 

 that in an electric field into which a different dielectric, is placed. This is the 

 analogue of a non-uniform specific inductive capacity. So far the ground is pretty 

 safe. The succeeding remarks must be regarded as speculative. 



Return to the case of the two separated surfaces A and B ; what will happen 

 if they be connected by a third surface^say, to fix our ideas — a long, narrow cylinder 

 or wire ? It is not so easy to say certainly what the exact rearrangement will be. 

 It would seem A^ery probable that the hollow filaments near the wire would at once 

 be replaced by circulation round the wire. In this case the solid filaments must 

 rotate as a whole round it, while at the same time, although this has not been 

 proved, they will contract, their ends move along until they come together, form a 

 ring and get mixed up in the surrounding fluid. This would correspond to the 

 current of discharge. But notice that if this is so the fluid outside the wire 

 rotates round the wire while a stationary state is being reached, and, as we Icnow 

 that the field round a current is a magnetic field, it follows that the analogue of a 

 magnetic Jield is ajlow along the magnetic lines of force. 



It may, however, be farther necessary that these streams must contain vortex 

 filaments moving with the fluid, their axes at every point being perpendicular to 

 the lines of force there. 



If the field is determined by the flow of fluid independently of the vortex 

 motion it is not easy to see the hydrodynamical analogue of the induction of 

 currents when, say, a wire moves across a uniform magnetic field. If, however, the 

 vortex filaments as above are a necessary adjunct, it is conceivable how a movement 

 of the wire across the lines of flow and amongst the filaments produce an instan- 

 taneous disarrangement which will travel through the vortical fluid with the 

 velocity of propagation of transverse vibrations in a vortical fluid. That is in the 

 ether as Sir W. Thomson has shown with the velocity of light. 



It is diflicult to see any analogue to the rotation of the plane of polarisation of 

 light by a magnetic field. 



6. On a Diffusion Photometer. Bij J. Jolt, M.A., B.E. 



This photometer in its original form was brought before the British Associa- 

 tion in 1885 by Professor Fitzgerald for the author. The improved form, made 

 of translucent glass, is now shown. A full account will be found in the ' Philoso- 

 phical Magazine,' July 1886, p. 26. 



7. Third Report of the Committee on Electrolysis. — See Reports, p. 339. 



