APPLICATIONS TO MECHANICS 95 



In the case of waves which travel in an onward 

 direction, billows, and breakers, the molecular move- 

 ment is different. For instance, in one photograph, 

 taken after the sudden immersion of the cylinder, the 

 surface molecules are seen to describe parabolic arcs 

 in planes parallel to the direction taken by the waves. 

 The deeper they are in the fluid the less definite are 

 the curves described, and sometimes the direction of 

 the movement is almost in a straight line. AVhen the 

 cylinder is moved with a to-and-fro movement, the 

 molecules describe curves on the surface of the fluid 

 which are complete rings. In all such experiments, 

 the nature of the impulse imparted to the water so 

 modifies the character of the movement that in order 

 to obtain exact results the force must be applied by 

 mechanical means instead of by the hand. 



Currents and Eddies.— Owing to the circular form 

 of the tank, it is possible to set up a continuous flow 

 by means of a small screw immersed in the water. 

 The latter, however, must be placed out of sight of 

 the observer. The little bright beads will participate 

 in every movement set up in the water. 



The chronophotographs will, at any given moment, 

 show the successive positions of these shining bodies, 

 which will thus serve as indices of the path taken 

 and of the velocity acquired by the various currents 

 set up. 



An obstacle, consisting of a sheet of glass, was 

 placed in the current, making an angle of about 4o° 

 with the axis of the stream. The glass was so 

 arranged that it touched the walls of the tank, and 

 only presented its edge to the chronophotograjDhic 

 objective. A photograph was taken during a period 

 of three seconds, and the number of images taken 

 was forty-two to the second. 



On examining this photograph (Fig. 64) it was 



