95 



It IS obvious that if it were possible to deduce from a study of the 

 surface temperatures alone the circulation existing at the time, and 

 that is likely to continue to exist for some time to come, the making 

 of estimations of future berg drifts would be rather simple. Exper- 

 ience shows that with the cooperation of passing vessels isotherm 

 charts can be constructed every 15 days which show in excellent 

 detail the picture of surface temperatures over almost the whole 

 of the region between the thirty-ninth and forty-ninth parallels 

 and the forty-third and fifth-sixth meridians. This embraces all 

 of the usual ice regions south of Newfoundland and a goodly area 

 of their surrounding waters as well. It covers an area between ten 

 and twenty times as large as that which two ships of the character 

 of the present ice-patrol vessels could keep properly mapped dynami- 

 cally in the eddjdng waters about the Grand Banks, even if they 

 devoted alternating at-sea periods entirely to oceanography. 



The making of the isotherm charts from the ship's log and from 

 the received radiograms is a routine process that requires but one 

 or two hours of work per day from two men. It does not consume 

 any of the ship's scouting time or interfere with any other of her 

 ice-patrol duties, and so is entirely within the capabilities of the 

 present ice patrol. On the other hand, the full projection of dynamic 

 current maps is not, as will be shown in the next section. 



The vital question is whether the best surface isotherm charts 

 procurable can be interpreted in any way to be relied upon to prop- 

 erly picture the circulation of the layers of water which control 

 the movements of the bergs. Hardly less important is another 

 query: If the true circulation at the instant is mirrored properly, 

 how much can the surface isotherms be depended upon to tell the 

 story of what currents will prevail from two to seven days later? 



The first thing that must be done when approaching the problem 

 is to determine what paths icebergs have taken in the past. The 

 main courses that they are likely to follow in the southern part of 

 the Labrador Current are very clearly shown in Figure 30 on page 

 69 of Coast Guard Bulletin 16. This chart, which sums up the 

 berg-drift information gathered b}^ the ice patrol up to and through 

 the 1927 season, is one of the fundamental sources of practical ice- 

 drift information. It is constantly referred to when bergs are reported 

 or sighted because it gives some information of the direction in 

 which the ice may drift while not under surveillance. This chart 

 and its companion chart, Figure 29 on the page facing it, when 

 compared with the composite picture obtained from a study of all 

 the isotherm charts explain why it has been assumed that the bergs 

 tend to remain in the colder waters and to follow in general the usual 

 paths taken by the varying pushes of the same. 



