92 MOVEMENT 



regular oscillations to the water. These rhythmic 

 movements should be in that part of the tank which 

 is furthest removed from the point of observation. 

 The lens of the camera should be left permanently 

 open, so that the bright line may leave a track 

 corresponding to all the positions assumed, but with 

 greatest intensity where the velocity is least, that 

 is to say, in the immediate neighbourhood of the 

 dead points which correspond to the crests and 

 troughs, for here, just before changing its direction, 

 the movement is at a minimum. 



*If one wishes to have a better appreciation of the 

 changes in velocity during the different phases of a 

 simple oscillation as represented by the contour of 

 a wave, recourse must be had to chronophotography. 

 In other words, the admission of light must be very 

 brief, and the intervals of time perfectly regular. The 

 successive positions of the level of the fluid will 

 thus be obtained. These positions will be represented 

 as curves, which will be further apart at the centre 

 of the oscillations and closer together in the neigh- 

 bourhood of the crests and troughs. If the rhythm 

 of the movement is changed by gradual acceleration, 

 another variety of " chopping " commences, in which 

 the waves are shorter. In each case the profile of 

 the wave, as it passes along in crests and troughs, 

 takes the form of " trochoids," a name invented by 

 those interested in hydraulics. Waves moving onward, 

 billows, and breakers, can be taken by chronopho- 

 tography, so as to show the speed at which they 

 travel, as well as the changes in size and shape which 

 occur in them. 



We took a photograph of a wave by disturbing 

 the water in the tank in the following way : — The 

 cylinder described above was immersed in the water 

 just to the right of the transparent part of the tank 



