664 



Professor George Chrystal 



[May 17, 



now start a seiche in the long tank.* This is done by stirring horizon- 

 tally in the middle of the tank, with a period of about 5 sec, which 

 depends on the shape of the basin in which the liquid is confined, and 

 on gravity ; but on nothing else. The result, you see, is a motion 

 of quite another kind, which we call a pure uninodal seiche. It is 

 a periodic wave motion ; but the wave form does not travel as in the 

 two former cases. At first sight it would appear as if the surface par- 

 ticles merely moved vertically upwards and downwards, except at one 

 point, which we call the node, where there is little or no perceptible 

 vertical motion of the surface. In reality the water particles describe 

 rectilinear orbits of various lengths, inclined at various augles to the 



Fig. 5. 



horizon. These are drawn to scale for a selection of different par- 

 ticles in the lantern-slide which is shown (see Fig. 5). We can show 

 the nature of the motion at various places by dropping in red ink as 

 before. The whole of the water is noAv in motion ; and the striking 

 thing is that all the water particles keep exact time, like a company 

 of well-drilled soldiers. Each particle is at the middle of its orbit at 

 the same time ; each at the arrow-marked end at the same time ; and 

 so on. This collaboration we express mathematically by saying that 

 the particles are always in the same phase, although the directions and 

 lengths of the orbits vary from point to point. It is a matter of 

 wonder that this should be the case for two particles 12 feet apart in 



* The dimensions of this tank are: length 12 ft., breadth 2a in., depth 

 12 in. It was fitted with a wooden parabolic bottom, concavity upwards, 

 the parabola reaching within 2 in. of the top and bottom. The stirring was 

 effected by means of wooden paddles, slightly less in breadth than the trough, 

 reaching nearly to the bottom, and working about horizontal axles resting on 

 the upper margins of the trough. 



