INFLUENCING FACTORS 147 



one caused by swells. On the other hand, a rip current may accentuate 

 the irregular and tumultous nature of the surf if this is due to storm 

 seas. 



Bail and ice. — It is common knowledge that a hail storm tends to 

 knock down a sea, and hence to reduce the surf, even though the wind 

 may be high. And loose pack ice, so kills a sea that a vessel following 

 the lanes through it may be sailing in smooth water (fig. 43), even 

 when a gale is blowing and the waves are breaking heavily against the 

 exposed edges of the pack. During the days of sail, many a whaler 

 found a safe natural harbor from storms in this way. When ice 

 crystals form in the water, they smooth the waves much more efficiently 

 than a film of oil would, so greatly does their presence increase 

 the internal friction among the water particles even before they be- 

 come solidified. We have, for example, seen at Barnstable Beach, 

 Massachusetts, a moderately heavy sea (wave height estimated at 4 to 

 8 feet) that was breaking offshore, under a winter gale, so smoothed 

 out in its passage through the last 100 feet or so of water soupy with 

 ice crystals, that it simply rose and fell against the 5- or 6-foot ice 

 barrier without breaking at all. Snow falling on the water has a 

 similar mechanical effect, though in a lesser degree. 



Marsh grasses and seaweeds. — A tall growth of marsh grasses may 

 so obstruct the waves in an estuarine situation and on a marshy fore- 

 shore, during the stage of the tide when it is partially submerged, that 

 a belt a few yards wide may wholly prevent breaker formation, even 

 in weather when there is considerable surf on neighboring stretches 

 of beach that are not protected in this way. We think in particular 

 of a certain gently sloping beach between rocky headlands on the 

 south shore of Boston Bay, where we have often landed our dory with 

 ease through a patch of partly submerged marsh grass (Spartina), 

 an acre or less in extent, at times when the surf would have made it 

 difficult for us to have done so on the bare beach close by. 



A thick growth of seaweed, the fronds of which are long enough 

 to reach to the surface, may afford considerable protection to an 

 exposed anchorage, as is said to be the case at Kingston, an open 

 roadstead in Lacepede Bay, South Australia, where ships loading 

 with wheat can lie safely during northwest storms, though a heavy 

 surf develops along the neighboring coast. 42 Similar conditions no 

 doubt exist on a small scale elsewhere. 



Darwin long ago stated that beds of giant kelp (Macrocystis) , 

 which may reach to the surface from depths as great as 45 fathoms, 

 make excellent natural breakwaters in the Straits of Magellan, re- 

 marking that waves from the sea soon decrease in height and are 



42 Interesting accounts by ship captains are to be found in Nichelson, J. G. 1888. 

 Aus dem reisebericht der . . . "Franz." Ami. hydrogr., Berlin, vol. 16, p. 22. 



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