350 CAPILLARY ATTRACTION, ETC. [MEMOIR XXVI. 



by induction. The conjoined effect of the forces thus 

 brought to bear on the needle causes it to move on its 

 centre, and take up a position of rest between the polar 

 wires. To this, if it were turned aside, it would return 

 after a few slow oscillations. 



From this it appears that though the polar wires are 

 plunged into a conducting medium and the current is 

 passing, they still act as centres of attraction. 



When a globule of mercury under the surface of some 

 water is brought in presence of a point of attraction, p, 

 situated at a short distance from its surface, two tides 

 will be formed on the globule ; one, a, 

 Fig. 76, directly in front of the point 

 of attraction, the other, 5, 180 from it. 

 In the quadrantal regions, c, d, there 

 will be an ebb. If the point p be 

 moved round the globule, both tides will follow it, keep- 

 ing the same relative position that they had at first. 

 These motions imitate on a small scale the effect which 

 takes place by the action of the sun and moon in pro- 

 ducing the tides of the ocean, and the explanation is 

 the same as in the case of those tides. 



If a spring-tide were formed on a spherical ocean, and 

 the sun and moon then annihilated, the elevation must 

 sink, pressing the under waters aside, and causing them 

 to rise where they were depressed. But the motion 

 would not cease when the level was reached, for the wa- 

 ter would arrive at that position with an accelerated ve- 

 locity. It would, therefore, pass that position and form 

 a high water w r here it had been low, and low water 

 where it had been high. And this would be repeated 

 again and again. 



Now this theoretical case may be imitated with the 

 globule of mercury, for on approaching the positive wire 

 to it, a position will be reached at which contact will 



