84 



It is easizT to study tidal currents thnn oc _:an drifts bacause 

 most of the work can be don^ noar land, in shallow, and often within 

 enclosed wat-rs. Undar such conditions the direction and speod can 

 be measurf.d directly from hour to hour as th- tide ebbs and flows by 

 current meters, by chip-log, or by float. And an enormous amount of 

 this work has bion don? by the tid?l service of the different 

 countries, including continuous observations ov-r p^riods of manv 

 weeks or months at strate<Tic locations (lightships, for instgnce). 

 But while th- stag; of the tide can now be predicted in advance for 

 any time of the day with great accuracy for most of the iraport<int 

 harbors of the world, in few c-jses (and perhaps nowhere off op-.n 

 coasts) is it yet possible to do this for the velocity or precise 

 direction of the current, because the latter is so often complicated 

 by the wind, and by whatever non-tidal drift may dominate the 

 particular region in question. 



Rapid advances are being made in current work alono- the coasts 

 and in the harbors of the more important maritime nations. Thus the 

 U. S. Coast and Geodetic Survey is making comprehensive current 

 surveys of the more important harbors and waterways of the United 

 States. But this is necessarily slow work with the appropriations 

 available. Hence present knowledge is in many frequented waterways 

 insufficient-witness the necessity the engineers for the projected 

 tidal -power developm'.nt in Passamaquoddy Bay have been under of making 

 their own survey of the strengths and directions of the tidal currents 

 there. And for administrative reasons the coastal and tidal surveys 

 of the different governments are seldom able (never able in the case 

 of the United States) to extend this work beyond their own coasts or 

 those of their dependencies. It follows that there are sjrious gaps 

 in our knowledge of the tidal currents around the shorelines and in 

 the bays and estuaries of all the countries that are more backward in 

 this respect. And what is known of such regions has necessarily been 

 gained more or less haphazard, as opportunities offered for some man- 

 of-war or other ship to take current measurements while on foreign 

 station. The paucity of detail as to the direction and velocity of 

 the tidal currents given in the sailing pilots for the South Atlantic, 

 for the Central Pacific, and for the Eastern Archipelago will make 

 this clear. Here a wide field lies open for oceanogra.phic research, 

 where knowledge gained will sooner or later be of practical advantage 

 to the navigator. 



B. SOUNDINGS 



Knowledge of the topography of the sea bottom, i.e. of the 

 depth of the water along the coast, is, to the navigator, as impor- 

 tant as is the detailed charting of the coastline itself. Not only 

 does his ability to enter harbors in safety depend on this knowledge, 

 iBut by sounding he can feel his way, and often can locate his position^ 

 when fog or storm hide every visible land mark, terrestrial or 

 celestial. 



Until very recently it has chiefly been in comparatively shoal 

 water, say less than 100 fathoms, that soundings have been helpful 

 to the navigator, and the importance of mapping the depth near land 

 in the greatest possible detail has so long b; en fully appreciated, 

 and so much effort has been devoted to this, that existine- charts 

 leave little to be desired for navigational purposes, for the more 

 frequented coasts. 



