6 The Sea 
side are mapped from aerial reconnaissance photographs. 
The German exploring vessel Metear has been particularly 
active along these lines, and has published an album of wave 
pictures, copies of which you may find in a large library. Such ~ 
photographs show that although actual waves are often ap- 
proximately the shape of the trochoid, they may often have 
extremely irregular shapes. We suppose that this irregularity 
is due to the fact that at sea there are usually many waves of 
different lengths and sizes, and that when they run together 
at random, odd shapes inevitably result. A good example of 
how much a simple wave motion may be modified by such a 
process of interference, as it is technically called, is to be seen 
in the “cross seas” where one train of ,waves crosses another 
at an angle. This phenomenon of interference that causes the 
irregularity of ocean swells is well worth reading about in an 
elementary physics text. It also causes such diverse phe- 
nomena as the throbbing of deep organ notes, the iridescent 
colors of oil on water, and contributes to the twinkling of 
stars. : 
Swells are usually started by the action of the wind, and 
may continue long after the wind that has caused them has 
died down. Of course, as we might expect, they always go a 
little slower than the wind which caused them. 
HOW FAST DO WAVES TRAVEL? 
Ocean waves travel at different speeds. There is a well 
established relation between their size and their speed. This 
relation is different for waves in deep water and those in shal- 
low water. In deep water the speed of the waves depends 
upon their length (that is, the distance measured from the 
crest of one wave to the crest of the next). No matter how 
