PROPMEEIOM Hie SU BAOULOUS SHORE LERRA CE ~13 
direction of wave movement, by a process which has something 
in common with walking. In this way also, pure oscillation 
would give rise to a current. 
It is evident then, that when a system of waves of pure 
oscillation advances over a shallow bottom, any supposition that 
may be made regarding the adjustment of internal movements 
will result in a forward flow of water above, and a dominance of 
movement in the opposite direction below. Owing to friction, 
the latter alone is never equal to the former. The resulting 
movement of water in the direction of wave propagation, whether 
it be viewed as a current or as an increase of the positive over 
the negative parts of ordinary waves, is not the same as waves 
of translation, technically so called.* These latter obey differ- 
ent laws and move with different velocities. They may be 
occasioned by breakers, or may perhaps grow out of oscillatory 
waves by gradual transition, but their movements are character- 
ized by certain features to be mentioned later. 
The return current.—As soon as a current is initiated a return 
of the water becomes necessary. If the process described above 
be supposed to take place on a shoal without shores this return 
may take place by another route. In this case the current may 
proceed as described for an indefinite time. If there is no 
return over another area by horizontal circulation, then the return 
must be over the same area by vertical circulation, that is, either 
above or below the original current. If the forward orbital 
movement above exceed the backward orbital movement below, 
as seems necessary, and no lateral escape is at hand, the pressure 
due to increased height of the water would cause a counter cur- 
rent which would appear below as undertow. 
Action on bottom materials—The essential value of the consid- 
eration of these currents, springing from waves of pure oscilla- 
tion, is in the necessary conclusion that the work of such waves 
7s backward at the bottom, and not forward. The advance of the 
water described is due to interference with its backward flow. 
The same friction which impedes the backward movement of the 
tJ. ScoTr RussELL, The Wave of Translation. 
