99 



the densit}' wall is, of course, unknown. Nevertheless, in cases 

 where a cold wall is strongly developed, it appears that the following 

 rule holds good. An observer stationed along the junction line 

 between markedly warmer and colder water, will, if he faces directly 

 toward the cold water and puts the warm water at his back, have the 

 local ocean current in his immediate vicinity, both behind and before 

 him, flowing from his left-hand side toward his right-hand side. 



This is not advanced as any general rule for the Northern Hemis- 

 phere- or even as an infallible one for use in the whole of the 150,000 

 square sea mile most probable ice area about the Grand Banks. 

 At some times and in some places the varying relations between 

 different water masses are affected by earth rotation, wdnds, shoals, 

 and other factors too much for that. There are complications caused 

 by shifting of surface and subsurface layers from their original posi- 

 tions, and masking effects due in place to late season surface warming 

 over true cold water. Nevertheless, the rule is a good working one 

 for the ice patrol when it is used with due regard for the special 

 dynamic and other circulation conditions that are liable to be 

 encountered. 



When the horizontal surface temperature gradient is small, and in 

 the localities liable to subsurface pushes of cold water the case is not 

 clear. Nevertheless, the surface isotherm charts are already of such 

 great value to the patrol for the estimation of berg drifts that every 

 effort should be made to improve their character, regardless of whether 

 or not the dynamic oceanographic maps are continued. 



It is certain that the isothermal method is susceptible of greatly 

 increased development, mainly through increasing the number of 

 incoming water-temperature reports. Such an increase in number 

 would be very desirable even if the values were not useful for con- 

 structing surface isotherm charts. These reports serve to keep a 

 close check on the positions and courses of the different passing 

 vessels, inform the patrol what areas are being searched for ice, and 

 tell what weather conditions are prevailing in different localities. 



An average of 60 water temperature reports per day were received 

 during the 1929 ice season. If this could be increased to 100 reports 

 per day the value of the isotherms for estimating currents and berg 

 drifts would be more than doubled. The value of the isotherm charts 

 would be greatly increased by a few more reports per day if these 

 could be obtained from vessels crossing the ocean between the United 

 States and the Canadian tracks. Requests for water temperatures 

 from this area are often inserted in the broadcasts in order to obtain 

 more values from the little-frequented parts of it. An increase" in 

 the total number of temperature reports and a little better distribution 

 of them would permit the making of a valuable isotherm chart 

 weekly instead of every 15 days and would also permit a slight 



