86 



ice season on a day when refraction was making objects loom iip' 

 higher than usual the white ice-patrol vessel was reported to herself 

 as a berg by a steamer that was observed to be passing by at a distance 

 of approximately 10 miles. 



However, if after a careful inspection no flaw in an incoming iceberg 

 or temperature report is apparent, it must be accepted as correct and 

 plotted on the cruise chart. The plotted bergs always have the date 

 of the report written near by so that their successive positions from 

 day to day can be followed. The water temperature values are plotted 

 in different colored inks, the colors being changed every few days to 

 enable the special portion of the cruise period during which they were 

 received to be readily seen. 



The water-temperature values and the ice, but especially the latter, 

 are never looked upon as something fixed and unmoving. They are 

 constantly regarded as though in motion, and from the moment they 

 are plotted they are looked at with a continually questioning attitude. 

 Where will they be and how much will they be changed in one, two, 

 or more days? If the patrol is to be of high value to shipping this 

 attitude is necessary. Bergs often have to be found several days 

 after they have been reported, as at the end of a period of fog, or 

 after the patrol has been released by the complete melting of a par- 

 ticularly dangerous berg that was being watched. Constant esti- 

 mates must be made of berg positions at the expiration of different 

 time intervals. The patrol has time to search out only the most 

 probable locations of reported bergs. In most cases, due to the 

 normal prevalence of strong currents, it can afford to go directly 

 toward the spot where a berg w^as last sighted or reported only when 

 the information is extremely recent, say, less than one day old. 



The task of keeping track of the southern ice limits would be fairly 

 simple if good visibility were as prevalent over the Labrador Current 

 as it is over the Gulf Stream 180 miles south of the Tail of the Banks. 

 As it is, the patrol has to keep up a constant matching of its wits against 

 fog-shrouded currents. Its errors and successes are strongly brought 

 home as its searches during daylight periods of good visibility are suc- 

 cessful or fruitless. There is a constant stimulus to find new ways of 

 predicting berg movements and to perfect the old, not so much for the 

 information of shipping, but for the guidance of the ice searches of the 

 patrol vessels themselves. 



As the bergs are little affected by the wind, their movements are 

 mostly controlled by ocean currents. Therefore, any method of 

 predicting ice movements entails finding out the drift of water at the 

 time. The present state of knowledge regarding ocean physics 

 about the Grand Banks makes it appear that hydrodynamic surveys 

 are the best means for determining the oceanic circulation there. 

 The ice patrol has studied and worked with dynamic oceanographic 



