INFLUENCING FACTORS 149 



smoothed out in their advance through the weed. 43 Waves, however, 

 that advance through narrow beds of giant kelp that are separated 

 from the shore by zones of open water, as is the case at many points 

 along the Pacific coast of the United States, may still cause a surf, 

 the severity of which depends partly on the breadth and density of 

 the floating barrier in question, but partly on the original shapes and 

 dimensions of the waves, because storm seas are dampened down to 

 a greater degree than swells are, in their passage through the kelp. 



Dense beds of the so-called "eelgrass" 44 combatted the development 

 of breakers in much this same way, in many shallow estuarine situa- 

 tions, during stages of the tide when its leaves floated up to the top of 

 the water, until a parasite almost exterminated this common plant far 

 and wide, not only along the northeast coast of North America, but 

 along the northwest coasts of Europe as well. The rock weeds (Fucus, 

 AscophyUum) may act similarly, though on a very small scale, around 

 the more protected sides of some offlying ledge at low tide, when 

 floating fronds may tend to dampen down the waves. It may then be 

 possible for collectors of seaweed to work there, or for crabbers or 

 anglers to land, until the tide rises, when the fronds of the rock weed 

 become submerged so deeply that they no longer afford any protec- 

 tion. 



Patches of floating "gulf weed" (Sargassum) in the so-called "Sar- 

 gasso Sea" have a similar effect on local waves, as we have often ob- 

 served. But gulfweed does not occur coastwise in sufficient quanti- 

 ties, nor are the drifting beds of it stationary enough, to be of any 

 importance in the surf problem. 



Oil. — The calming effect on waves of a film of oil has been known 

 for ages ; Pliny, for example, mentioned it in the year 77 A. D. And 

 every work on seamanship includes directions for its use in storms. 

 Briefly, oil extinguishes the smaller wavelets, and — its especial value — 

 prevents the waves from breaking. It is not clear just how this 

 is brought about, for the older view that its effects are due to lessened 

 friction with air seems not to be correct, while it is doubtful whether 

 more recent explanations, based on the relative viscosity and surface 

 tension of oil as compared with water, are adequate. But in any case, 

 a film of oil spreading over storm seas, so lessens the grip of the wind 

 on them, that it immediately causes them to assume the character of 

 swells, and thus renders them much less dangerous. Oil, however, 

 does not interfere appreciably with the development of surf, for in this 

 case it is not the immediate effect of the wind that is responsible for 

 the formation of breakers. But oil may be of real assistance if it is 



43 Darwin, Charles. 1871. Journal of researches into the natural history and geology 

 of the countries visited during the voyage of H. M. S. Beagle round the world. N. Y., 

 p. 240. 



44 Eelgrass (Zostera) is a flowering plant in reality, not a true "seaweed"' or alga. 



