8 N. M. FENNEMAN 
increased friction which comes at length to consume all the 
energy derived from the wind which cannot then further increase 
the height of the waves. The opposite effects are seen when the 
wind has ceased. Friction gradually diminishes the differential 
movement of particles and the size of the orbits. Waves then 
become lower and at the same time longer in proportion to their 
height. 
Periodical large waves.—The change of wave-length must be 
propagated downward gradually. If such propagation were 
immediate, the wave-length at the surface would always be 
equal to that below. Not being immediate, there may be at 
times considerable differences in length. The periodical large 
waves always seen in a storm, may result from composition of 
lower and upper waves having different periods, as well as by 
composition of surface waves of different systems. 
WAVES IN WATER OF FINITE DEPTH. 
Wave base.-—The extent of orbital movement decreases in 
geometrical progression with depth. A point is therefore reached 
where the force is too small to overcome the viscosity of the 
water. Before this point is reached and at comparatively small 
depths the movement is so slight that it cannot affect the small- 
est solid particles resting on the bottom. This level, below 
which the largest waves are inoperative, has been called wave- 
base. Its depth for any given lake or part of the ocean is a func- 
tion of the height and length of the largest waves. 
Behavior of water above wave-base in pure oscillation.— Before 
considering the action of water on a bottom which lies above 
wave-base it will be convenient to examine its behavior at any 
horizontal plane passed through a system of waves. Referring 
to Fig. 9, let AB be such an ideal plane. Being above wave- 
base it is in the region where the “planes of continuity” (planes 
including always the same particles which are in a horizontal 
plane when at rest) are in trochoid curves. The lines of like 
phase are inclined toward the crests; hence the layer of water 
included between two planes of continuity is not only thinner 
