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methods for some years. It has been found that the makmg- of satis- 

 factory surveys of this sort entails steaming over lai^e areas fre- 

 quently, for the precise surface and subsurface information that must 

 be obtained can only be gathered by the patrol vessels themselves. 

 With only one ice patrol vessel out on duty at one time this interferes 

 greatly with the normal conduct of the practical part of the service. 



The second-best method of determining currents and berg drifts 

 about the Grand Banks is the interpreting of surface isotherm curves 

 in the light of past experience. The cooperation from shipping that 

 has been worked up enables the patrol vessel to plot the surface iso- 

 therms over a large area. The requirements of the hydrodynamic 

 method and the dependence which can be placed on the surface iso- 

 therms are both discussed in the next chapter. 



Conditions under which the patrol vessels work vary from month 

 to month and from year to year. Weather conditions and informa- 

 tion about ice coming in by radio often make it wise for the patrol to 

 alter its course of action several times during even a single day. No 

 hard-and-fast rules can ever be laid down, but existing circumstances^ 

 as viewed in the light of the patrol's past experience, must always 

 determine the cruising and other activities of the patrol vessels on 

 each cruise. As a general principle, however, it can be said that the 

 patrol must leave for other vessels to find and report all ice east of 

 the forty-seventh meridian and north of the forty-fourth parallel. 

 To narrow down and simplify the problem still further, it can be 

 stated that the patrol vessels should almost exclusively confine their 

 own activities to the area of an equilateral triangle with sides about 

 175 sea miles long and corners near 41° 30' N., 47° 00' W., 41° 30' N., 

 51° 00' W., and 44° 48' N., 48° 00' W. 



The above triangle contains about 13,200 square sea miles, and so 

 is of the order of size that theoretically can be searched for ice two 

 times during each patrol cruise. The areas along the western and 

 southern edges of this triangle are most critical and should never be 

 left unguarded for more than a very few days at a time, even during 

 light ice periods. The reason for this is because the general drift of 

 ice is southward down the western part of the triangle, then eastward 

 along the westbound B steamship tracks near the southern part of it, 

 and, finally, northward and northeastward away from the southern 

 tracks and across the eastern side of the triangle. 



A berg entering the circulation at the northern apex of the critical 

 triangle, if it is not melted, shunted off by local currents, or held up 

 by some means such as grounding or coming into an eddy, can easily 

 be discharged into a very dangerous position right along the westbound 

 B tracks within a period of from 10 to 15 days. Fog and strong cur- 

 rents and the necessity for standing by the most dangerous bergs, 

 whether they are inside or outside of the critical triangle, make the 



