100 



reduction in the temperature interval between some of the isothermal 

 lines. 



Any final general statement at this time of the value of the 

 isothermal method of determining ocean currents in the Grand Banks 

 region would be premature, however. The whole matter of correlating 

 berg drifts and surface isotherms is now under critical study. The 

 drifts of bergs during the 1930 ice season will be closeh^ watched for 

 instances of their adherence or nonadherence to the isothermal indica- 

 tions. They should furnish considerable additional data upon which 

 to judge the merits of the case. 



B. DYNAMIC CURRENT MAPS AS BASIS FOR PREDICTION 



Another method, distinct from the examination of surface isotherm 

 curves by experienced observers, has been suggested for prediction 

 of iceberg movements. It involves the construction of dynamic 

 current maps, and is undoubtedly accurate when certain conditions 

 are fulfilled. To make good current maps of this kind, however, a 

 more detailed d^'namic mapping of the area is necessary than the ice 

 patrol can usually undertake. Therefore, under the present condi- 

 tions it is impossible to get the maximum obtainable amount of benefit 

 from this method. 



How an exact knowlege of the temperatures and salinities prevailing 

 at the different levels throughtout an area permits the construction of 

 a current map showing the local oceanic circulation has been fully 

 explained in the Coast Guard bulletins describing the ice patrols of 

 1926, 1927, and 1928. Coast Guard Bulletin 14, December, 1925, 

 gives detailed information telling how such maps can be made and 

 interpreted. Some idea of the effort which the ice patrol must put 

 forth to do satisfactory dynamic oceanographic work will be obtained 

 from the discussion which follows. 



Because of rapid mixture of warm Gulf Stream and cold Labrador 

 Current water, the ocean currents off the Tail and the eastern slope 

 of the Grand Banks are in a particularly active state of turmoil and 

 fluctuation. Therefore, to be sufficiently comparable, dynamically, 

 to give satisfactory current maps, the different oceanographic stations 

 within an area about 100 geographical miles square must be occupied 

 during a period of not over 10 days. The results would be more 

 reliable and lasting if the stations were taken within a period of 

 seven days. To confine the station work to an area smaller than 

 100 sea miles square, or to an equivalent rectangle of less than 10,000 

 square geographical miles, would be to limit so seriously the scope 

 over which the berg-drift predictions could be made as to make the 

 current map of very small practical value. 



The time during which the taking of a series of stations can be 

 safely spread out will vary much with the rate of change of location 

 of the different water masses in the area under observation. The 



