PROFILE OF THE SUBAQUEOUS SHORE TERRACE 19 
than that of waves of oscillation, when like wave-lengths are 
assumed, the two rates having about the ratio of three to two ;* 
(4) the water particles move forward and not backward, start- 
ing from rest as the wave approaches and coming to rest when 
the wave has passed; (5) the forward motion of particles at all 
depths is the same and equal to the volume of the wave divided 
by the depth of the water; (6) the paths of the particles are 
semi-ellipses in a vertical plane, the major axis being the dis- 
tance through which the particle moves forward, and the minor 
axis varying from zero at the bottom to the height of the wave 
at the surface. This movement is in no sense the same as that 
of wind-driven waves or any other oscillatory wave motion com- 
pounded with current. It usually coexists with the latter on 
shallow bottoms, resulting in waves of a hybrid kind; but waves 
of nearly typical translatory character may sometimes be seen in 
nature. Whether the waves be of a pure or mixed type, the 
essential fact here is that a new factor has entered, whose action 
at the bottom is different from that of oscillatory waves and 
from that of currents. 
The fact of this change to translatory character on a gently 
sloping beach may be seen in the behavior of floating chips 
which are seen to move forward on crests but not backward 
between crests. In place of the trough proper is a wide strip 
whose surface is almost flat and the water of which is standing 
still. The laws of translatory waves require that they move 
more rapidly than the oscillacory. This might be expected to 
reveal itself in broadening intervals between crests as waves take 
on the translatory character. It is probable that this may occur 
under suitable conditions. The tendency is usually more than 
counterbalanced by two factors. The first is the decreasing depth 
which is the main factor in controlling the velocity of waves of 
translation. The second is the increasing strength of undertow 
near shore which retards the translatory movement at the bot- 
tom. 
As to the manner in which this new habit is developed, it may 
 [bid., p. 288. 
