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BRITISH (CINEMATOGRAPHY
Vol. 23, No. 4
Fig. 3. Generation of a water-wave.
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Generation of a Cycloid.
in an Atlantic storm and some waves ^-mile long have been observed on occasion.
By anology with the trochoid, the height may be any fraction less than 1/7 of the length, as obtained by moving P towards the centre o{ the circle in Fig. 2. When P is at the centre, no wave results, corresponding to a calm surface.
The velocity with which the wave travels is related to the length of the wave by the relation
v /5H
where g is the acceleration due to gravity ; that is the velocity is proportional to the square root oi~ the length. Hence the longer the wave the faster it travels and vice versa, but there is a lower limit to velocity.
The waves we have considered so far are " gravit) waves," in that they owe their
Fig. 5. Sharpest water-crest in deep water.
form and propagation to the effect of gravity on the water particles, causing them to fall in their downward paths and opposing their upwards motion, as a pendulum is gravitycontrolled. It is also possible to produce waves which are controlled less by gravity than by surface tension. Surface tension is the property of a liquid whereby its surface exhibits elastic properties, as though covered by an elastic film. It is demonstrated by the formation of drops on a greasy surface and the curvature of the surface where it meets an immersed wall. Such waves in a liquid are