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of greater density stratification, and hence a retention in the upper 

 layers of heat absorbed from the sun's radiation. Variations of sur- 

 face temperature of the same order of magnitude are frequently en- 

 countered, however, in the total absence of icebergs. 



In fog, the use of the steam whistle or foghorn for detecting ice by 

 echo is of little value. Sound waves will be reflected only by a high 

 vertical wall, and are not always discernible. The absence of echo is 

 by no means proof that no bergs are near. 



During daylight, in the absence of fog, a vessel may usually pro- 

 ceed through berg-infested waters without deviating materially from 

 her course and with a minimum loss of time ; but in darkness or fog, 

 extreme caution must be used. In a thick, low-lying fog, especially 

 with a clear sky and during dark autumn nights, it is well for vessels 

 not equipped with radar to anchor or heave to temporarily, as under 

 such conditions there is no warning as one approaches bergs. A pro- 

 cedure that has been found helpful under these circumstances is to 

 locate all the bergs visible at sunset, and to lay down their position 

 relative to the ship on a maneuvering-board diagram. From time to 

 time, while lying-to in the darkness, the ship is steamed to windward 

 enough to make up for the estimated wind drift that has occurred. 

 Ship and bergs (which because of their deep draft are virtually un- 

 affected by wind) thus keep their relative positions. The navigator 

 by referring to his diagram can so regulate his allowance for drift as 

 to maintain a safe margin of distance from all the bergs. 



There are few things more dangerous than threading a fleet of bergs 

 in thick weather. The ship may suddenly encounter one of these 

 masses of ice, unseen through the fog and rain until it is close aboard. 

 There is danger in making fast to them in such weather, for they them- 

 selves are often in motion, and may carry the vessel upon a grounded 

 mass or a heavy floe; there is also danger from an overturn of the 

 berg, or a break-up or from the fall of overhanging pieces. A position 

 may be taken in the lee of large icebergs where a clearing may be 

 found, but a sharp lookout must be kept to leeward. 



When working through a berg-studded sea in low visibility, a 

 full-powered ship should reduce speed to a minimum compatible with 

 quick maneuvering. A sailing vessel or low-powered auxiliary will 

 drive rapidly to leeward if hove-to; these vessels should head-reach 

 and endeavor to hold a weather gauge on the berg last sighted until 

 the weather improves. If icebergs are sighted ahead in thick weather, 

 sailing vessels and low-powered ships should go about and retire to 

 the area of clear seas and wait for fair weather. Care should be exer- 



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