Shallow Water Wave Transformation through External Factors 139 



so easily with sea water, the circumstance that in nature the entire water- 

 mass of surf waves is interspersed with innumerable bubbles, and perhaps 

 also the fact that sea water is rich in plankton, apparently contribute to this 

 considerably different behaviour in the two cases. 



5. Calming Effect of Oil on Waves 



Substances which are either mixed freely with the water, such as mud, 

 ice, seaweed, eelgrass, etc., or cover the sea surface, like some oils, and even 

 fresh water, can prevent the regular development of waves and cause existing 

 waves to decay rapidly. The wave-stilling action of mud and ice is a well- 

 known phenomenon. The ice crystals formed by the freezing sea water dampens 

 the swell very rapidly. The wave energy is dissipated more quickly by in- 

 creased friction. The same applies to drift ice in polar seas which dampens 

 considerably the wave action and reduces the swell. Sea weeds and eel grass 

 in sufficient quantity can also prevent development of a strong swell. It has 

 often been observed that rainfalls have a wave calming effect. It should be 

 remembered that fresh water has a considerably lower density than the saline 

 sea water and that so long as it does not mix with the lower layers, it can 

 spread out over the surface in a thin layer. In this manner it has the same 

 effect as a thin layer of oil spreading over the surface. The wave dampening 

 action of the rain vanishes very soon because the difference in density rapidly 

 decreases through mixing. An old and successful way to calm large waves 

 in the stormy seas is to pour oil on the surface. Canvas bags filled with tow 

 or twist, can absorb a large quantity of oil, which is released drop by drop 

 through tiny holes punched into the canvas; this oil spreads with great 

 rapidity over the surface as an extremely thin film suppressing all capillary 

 waves and leaving only the long waves of the swell. It removes the breakers, 

 if the swell is not too high; the oil-covered surface becomes very smooth. 

 This smoothing action is completed within a few minutes (Grossmann, 1892). 

 Experience has taught that not all oils have such action and that viscous 

 heavy animal oils, like fish oil, cod liver oil, also rape seed or colza oil are 

 more effective than mineral oils; petroleum and soap solutions proved to be 

 useless. Even a small quantity like one or two litres of oil per hour is sufficient 

 to smooth a surface of 20-100 m around the vessel. There is no doubt 

 whatsoever as to the calming effect of oils on windsea and swell. 



The damping effect of oil has been associated for a long time with the 

 surface tension of the water and .of the damping liquid (Koppen, 1893). Each 

 wave motion tends to alternate the contraction and expansion of the surface 

 as can be rapidly deducted from Fig. 4. An expansion of the surface counter- 

 acts the surface tension. A smaller surface tension must, therefore, facilitate 

 the wave formation. The surface tension of sea water is approximately 78, 

 that of olive oil 37, or rape seed oil and cod liver oil 32 to 33, of petroleum 31 

 to 32 dynes, which is only about half that of pure water. The addition of 



