85 



An example of the accuracy of some of the older work is afforded 

 by the fact that charts of the Maldive Group in the Indian Ocean, . ' 

 based on soundings taken nearly a wentury ago are so accurate that we 

 found no appreciable error, in 1901-1902 except such as would natur- 

 ally result from subsequent growth or death of coral heads. Even off 

 the coasts of the northeastern United States, however, pinacle rocks 

 have recently been discovered, and surveys must be repeated at fre- 

 quent intervals off sandy coasts and inldts where bars shift, and 

 channels change. In fact, few laymen appreciate the extent of the 

 coasts where knowledge of the depth is still more or less imp-rfect. 

 For an example we need seek no further than the east coast of 

 Labrador where soundings are not only so few, but are so often 

 inaccurate, that a stranger must proceed with the greatest caution, 

 while considerable stretches of the coastline itself are still to be 

 filled in on the chart. In Alaskan waters employment of the "wire 

 drag" method has rc;cently added much important information, especially 

 as to the location of pinnacle rocks, such as ar:: apt to be overlooked 

 in oth-r kinds of surveys. 



Now that sonic methods of sounding have reached the stage of 

 practicability the application of measurimf.nt of depths to navigation 

 enters a new phase. In the first place it is now possible- to survey 

 a given ar-a much more rapidly than by thr old methods. In the 

 second, detailed information of the :-dges and slopes of the contin- 

 ents becomes more important, for as more and more of the larger ships 

 install sonic g-ar with which they can sound at any depth whil^ 

 running at full speed, they find it more and more helpful to pick up 

 the slope as an index to their distance from land, in thick weather. 

 Thus the Ice Patrol, during the season of 1928 found the sonic fatho- 

 meter of great assistance in navigating in the fog around the slopes 

 of the Grand Banks of Newfoundland. But at the same time the Patrol 

 cutters also found thst had these slop,-,s been better charted, they 

 could have placed much mor^ dependence on the positions indicated by 

 their own soundings. 



IV. CURRENTS AS AFFECTING HARBOR CONSTRUCTION, AND THE 

 PROTECTION OF SHORE PROPERTY 



We can only reiterate what was pointed out at the Conference on 

 Oceanography at the U. S. Navy fepartment in 1924, by General Edgar 

 Jadwin, that the direction of the current must be taken into account 

 in planning harbor entrances on sandy coasts in order that the 

 entrance jetties may be designed and constructed either to catch and 

 hold the drifting sand, or to divert the latter past the entrance so 

 as to prevent the filling of the channel with sand. The currents of 

 importance in this case are those close along the tide line; and at 

 the times when these are strong enough to drift the sand along the 

 shore, they may either be parallel to or opposite to the general 

 dominant drift off-shore, depending on the direction from which the 

 storm waves travel, and the angle et which these strike the coastline. 

 At the tip of Cape Cod, for example, the only storms that drive heavy 

 enough seas against the be«ich to move much sand are from the eastern 

 quadrant. Consequently, the beach-drifting is toward the west and 

 southwest, whereas the dominant movement of the water only a short 

 distance off-shore is in the opposite direction. 



In any given case, therefore, a more detailed knowledge of beach 

 drifting is requisite than has yet been gained for any considerable 



