1903.) HAUPT—DEEPER NAVIGABLE CHANNELS. 205 
formation and maintenance. It is important that a careful diagnosis 
be made of each case to ascertain its preponderating element. 
PHYSICAL CONSIDERATIONS. 
Thus it is seen that the physical agencies become of fundamental 
importance, and that a clear distinction must be made between bars 
formed from littoral drift and those formed from the detritus car- 
ried down by streams. For tidal inlets it is also important to ascer- 
tain the prevailing direction of the littoral movements which have 
frequently but erroneously been supposed to follow the prevailing 
winds, whereas it is more frequently found to be the resultant of the 
configuration of the adjacent coast line and of the angular wave 
movements, especially during the flood tide, when the waves are 
most heavily charged with silt. 
Knowing the direction of this general resultant, the engineer can 
then determine on which side of the channel his protecting work 
should be placed, although there still seems to be a radical differ- 
ence of opinion as to whether it should be on the near or far side, 
for only recently it was recommended that if a single jetty were 
built at a certain inlet on the Southern coast, it should ‘‘ be located 
on the south of the channel, since the drifting sands come from the 
north. At this place, however, while the drift is comparatively 
slow, it is an enormous sand bank which moves, and which always 
moves very positively in one direction, and it is difficult to see how 
such a constant force from the north could avoid crowding the 
channel close to the jetty.’ The jetty plan was therefore rejected. 
Frequent experience in the construction of two jetties, where the 
farther one has been built in advance of the nearer one, has served 
to show the fallacy of this location and order of procedure. 
The requirements to be met at tidal inlets are, free admission 
of the flood tide as the only source of ebb energy, protection of the 
bar channel from the prevailing direction of the littoral drift, con- 
servation of ebb tide as it passes seaward over a narrower path on 
the bar, development of its potential energy in useful work locally 
on the bar crest and an automatic adjustment to any stages of wind 
or tide. All of these may be better fulfilled generally by one jetty 
than by two, and manifestly at about half the cost. These results 
are rendered possible by placing in the way of the ebb current a 
curved resisting medium in such position as to maintain a contin- 
uous reaction along its concave face. In fine, this structure be- 
