102 



actions. Because the current speed and the mobiUtv of the ocean 

 swirls are of the order of twenty to forty times less than that of those 

 in the atmosphere, the current maps when once made are probably 

 good for about one week, though it is doubtful if the detailed current 

 map of the hypothetical 10,000 square sea-mile area made during the 

 course of 7 to 10 days, can be depended on very closeh^ after three or 

 four days from its completion, due to the length of time required to 

 develop it. 



Taking the most favorable arrangement of courses possible, to make 

 a good useful current map of the 100 sea-mile square area with 50 

 stations as outlined above, it is necessary to steam about 600 sea-miles. 

 At 8 knots, a high average speed for a scientific ship to maintain under 

 the conditions prevailing about the Grand Banks, this would take 75 

 hours. Add 50 hours to this for the time required for work at the 

 50 stations, and a period of 125 hours, or just over five and one-sixth 

 days, of intensive undivided work by a ship in midocean is seen to be 

 necessary to gather the data required for the making of a dynamic 

 current map of what is really a very small portion of the usual ice 

 area south of the forty-eighth parallel. 



During a large part of the station taking period good visibility 

 should prevail, for it is very necessary to insure accurate geographical 

 location of at least a number of the stations to keep the whole map 

 from being hazy and indefinite. In places near the edge of the Banks 

 radio bearings and sonic soundings serve to locate position rather 

 accurately at times, but in most places many and good celo-naviga- 

 tional observations are absolutely essential to satisfactory position 

 fixing. Observations can only be obtained during weather excellent 

 for ice searching. 



If the Yerj exacting oceanographic work inseparable from the con- 

 struction of detailed dynamic current maps is seriously prosecuted, the 

 primary object of the patrol, the location by scouting and radio infor- 

 mation of ice for the protection of shipping, must be neglected to an 

 appreciable extent. Each time, before commencing upon the cruising 

 necessary to the construction of a current map, the commanding officer 

 of the patrol ship must weigh the possible advantages to be obtained 

 from a dynamic current map against all features inimical to the prac- 

 tical program that such construction will entail. A current map made 

 at the expense of a vessel's disastrous collision mth ice which might 

 have been averted, but for the oceanographic work, would involve a 

 cost entirely too high to pay. A single patrol ship can either make 

 cm-rent maps or stay with the ice. It can not do both at the same 

 time with any degree of justice to either. 



Apart from their practical value to the ice patrol, dynamic current 

 maps have a permanent scientific vailue which is not interfered with 

 by delay in plotting them. A number of current maps in the Grand 



