Internal Waves 



523 



The tests made by Ekman in a wave tank have confirmed in every way 

 the theoretical explanation of the "dead water". Figure 214 by Ekman shows 

 how at a and b "dead water" waves are created by the towed vessel; at c, 

 with increased speed, the ship has freed itself from the "dead water". There 

 is only a slight denting in the discontinuity surface below the ship, but no 

 internal wave system. 



Fig. 214. Ekman's experiments on "dead water" waves, a and b, "dead water" waves 

 created by a vessel; c, the ship is free from "dead water" waves. 



When a ship is anchored and a series of observations is taken within 

 short intervals one observes variations in temperature and salinity of short 

 duration; especially if the discontinuity in the vertical distribution of the 

 density is well developed. These variations may be caused by internal waves, 

 but mostly the interval in which the series is made is not sufficient to prove 

 the wavelike nature of the disturbances. One needs continuous registrations 

 of temperature and salinity and such measurements have been made only 

 recently and sporadically. However, there is no doubt that the boundary 

 surfaces are constantly in a wave motion and a state of complete rest is ex- 

 ceptional. We will refer to this later on (p. 531). 

 (c) Internal Waves Having the Character of Long Waves 



If the wave length of the internal waves is long in comparision with the 

 total depth (h + h') of the two layers, one can write \/xh and \/xh' instead 

 of the cotangents in (XVI. 9) and one gets with sufficient accuracy: 



c 4 - c%h' + h)g + 1 1 - ^\ g*hh' = 



