PROPEL VOL Mil SO BAQKTEOUS SHORE TERRACE 17, 
breaker line for waves without wind is far up the slope from 
wave-base, it may move down indefinitely near to wave-base 
when the wind is active. 
Tendency of wave to recover form.—Suppose a system of 
oscillatory waves to advance toward a shelving shore until the 
wave-base intersects the bottom. One effect must be produced 
here regardless of qualifying conditions. Bottom friction begins 
and that involves increased differential movement of particles, 
which is accompanied by shortening and steepening of waves. 
This implies increased internal friction, which in turn, operates 
to decrease the orbital motion and therefore wave dimensions. 
In so doing it would take away the conditions of bottom friction 
and its results. The wave would then return to its deep water 
form. Thus there is a chain of consequences from the original 
interference at the bottom, which involves at first the change of 
wave form, but later a restoration, the final result being reduc- 
tion in dimensions only, suited to the diminished depth. Another 
decrease of depth must then be assumed if the wave be supposed 
to continue its contact with the bottom. Thus there ts a certain 
minimum slope for the bottom, upon which the waves may be propa- 
gated as a shallow-water wave. In so far as the wave is affected 
by increase of orbit due to diminishing amount of water, the 
effect will be to hasten the deformation and to retard the recov- 
ery of form. If the wind is active it would retard the decrease 
of orbital movement and the minimum slope mentioned would 
be smaller. 
Limit of tendency to recover form.—The greater the reduc- 
tion of depth, the greater the increment of internal friction 
tending to reduce the wave size, and the greater this friction, the 
more rapidly does it operate to accommodate the wave dimensions 
to diminished depth. This corrective tendency has, however, a 
limit. This limit is marked by the breaking of the wave. There 
as, therefore, a certain maximum slope for the bottom upon which the 
wave may be propagated without breaking; at or beyond this maxi- 
mum the wave breaks and other agencies come in. The effect 
of wind as before, is to diminish the maximum slope; hence 
