12 CAUSE OF THE SPIRAL MOTIONS. 



the other ; the mercurial tide falls with an accelerated motion, and the line which before 

 was the conjugate axis of the ellipse, now becomes the transverse, a tide being pro- 

 duced at right angles to the former one. But here the strict comparison ends, for, as 

 the mercury ebbs from its protuberant position, the metallic connexion breaks, and the 

 wire is again put in action as a point of attraction ; the motion of the ebbing tide is 

 checked ; it flows once more ; once more the metallic contact is complete, and when 

 the tide falls, it is only to flow again, as long as the battery current passes. Tides take 

 place at right angles to each other, in a series too rapid to be counted, and the whole 

 surface of the mercury is worked into those various and beautiful undulations which 

 have been before referred to. 



32. In endeavouring to ascertain the true cause of these phenomena, the French phi- 

 losophers were, I believe, the first to observe motions in the water or other liquid of 

 communication, as if a gentle wind played over its surface, bearing light bodies in its 

 vortex. The explanation of these appearances I here add, because no one as yet has 

 given it, and it affords an illustration of certain propositions delivered by Sir I. Newton, 

 in his Principia, concerning the doctrine of pulses in elastic fluids. 



33. We have hitherto been considering a globule of mercury as a substance mathe- 

 matically fluid. Such, however, in effect, it is not ; the water in contact with it pos- 

 sesses those properties in a much more eminent degree, so that, in comparison with it, 

 the mercury may be regarded as a solid resisting obstacle. Now, about a year ago, I 

 showed that when a voltaic current passes through a system such as this of mercury 

 and water, the capillary pressure on the bounding surface is changed ; but, if the at- 

 traction of the wire which is introduced into the water, and which is the ultimate cause 

 of this derangement, decreases in a duplicate ratio, it follows that this disturbance ot 

 pressure obtains only to a limited extent on the surface of the mercury ; or, in other words, 

 the excess of pressure produced by a voltaic current is not spent equally on all parts of 

 the mercurial surface, but those which are adjacent to the positive polar wire are more 

 affected than those at a distance. NEWTON has shown (Principia, v. ii., b. ii., pr. 41), 

 that if the particles of a fluid do not lie in a right line, a pressure propagated through 

 that fluid will not be in a rectilineal direction, but the particles that are obliquely posited 

 have a tendency to be urged out of their position. So the particles a a a a (fig. 9, pi. 

 1), pressing on the particles d b, which stand obliquely to them by reason of the shape 

 of the mass of mercury g, have a tendency to be urged from their places towards e 

 and c respectively, and the motion thus produced in a fluid diverges from a rectilinear 

 progress into the unmoved spaces ; and such a pressure taking effect on a liquid free to 

 move, continually returns the moving particles to their first position, after making them 

 describe an elliptical orbit. 



34. It has been remarked that the basis on which this explanation essentially rests 

 is, that a wire, from which an electric current passes, acts still as a point of attraction ; 

 an effect which involves the conducting and other electric properties of the system on 

 which the experiment is tried. Hence we gain an -insight into the cause of the paral- 

 ysis of these motions by the addition of certain substances ; the spiral motions going on 

 over the surface of the water have these explanations complicated with another con- 

 sideration, the figure of the mercurial mass. 



