APPLICATIONS TO MECHANICS 91 



from the under surface of the meniscus. On the 

 ground glass of the camera a very brilliant and fine 

 line may be seen marking the level of the water, and 

 which, moving with it, will imprint all the undulations 

 of the water on the photographic negative. 



Any internal displacement of the water can be made 

 visible by suspending small and brilliant objects in 

 the water, and illuminating them by the sun's rays. 

 For this purpose pieces of wax and resin are mixed in 

 the required proportion, the former being less dense 

 than water, and the latter of greater specific gravity. 

 From this solid material a number of small balls are 

 moulded, and then silvered over, in the same way that 

 pills are silvered by the chemist. These bright balls 

 should be slightly heavier than water, so that when 

 they are dropped in it they slowly sink to the bottom. 

 If a small quantity of salt water be afterwards added, 

 the balls gradually rise up and remain in unstable equi- 

 librium. A paper scale, divided into centimetres, may 

 then be gummed on to the side of the tank above the 

 level of the fluid. This scale, which will appear in the 

 photographs, will do very well to measure the extent 

 of the movements which are being photographed. 

 With such an arrangement a large number of experi- 

 ments with liquids can be carried out. A few of them 

 are here represented in the form of photographs. 



Changes in Shape of Fluid Waves. — The bright line 

 which marks the level of the fluid shows, on shaking, 

 variations in contour such as those afforded by 

 vibrating strings. The ventral segments and nodes 

 sometimes occupy fixed positions on the surface of 

 the water, as occurs in the case of a choppy sea. 

 Sometimes they advance with varying velocity, as in 

 rolling breakers. A similar chopping motion can be 

 set up in the water by plunging a solid cylinder 

 into the tank at regular intervals, and thus imparting 



