water. 



PHYSICAL OCEANOGRAPHY 275 



Sea 100 metres in height would manifest itself as a surface- 

 wave about 5 cm. high, that is, practically imperceptible, as the 

 wave is very long and proceeds slowly. Several of the 

 "Michael Sars " investigations indicate such boundary-waves, 

 but here also precise observations are lacking. They are, "Dead 

 however, known in one particular form, viz. as the boundary- 

 wave producing "dead water." When a comparatively fresh 

 and light water-layer, 2 or 3 metres thick, rests on a salt 

 and heavy layer, a passing ship may give rise to a boundary- 

 wave between the two layers. This wave may stop the ship, 

 so that it lies in dead water hardly able to move at all. Ekman, 

 who has investigated these phenomena, has demonstrated the 

 dead-water wave by the following experiment (see Fig. 185). 

 He put salt water, coloured dark, into a long basin, and on the 

 top he poured a thinner layer of fresh water ; when he slowly 

 towed a small model of a ship through the upper layer, a 



Fig. 185.— Ekman's Experiment to show the wave producing Dead-water. 



boundary-wave arose, as seen in the figure, which, when strongly 

 developed, checked the speed considerably. 



Naturally when a wave like this passes a certain spot on the 

 sea, the undulating boundary between the two water-layers will 

 at one moment be vertically nearer to that spot, at another 

 moment farther down. Similar vertical oscillations may 

 arise in other ways, as we shall now briefly indicate before 

 describing some observations made during the cruises of the 

 " Michael Sars," which prove that such undulations do exist 

 in the sea. 



We may first mention one of the effects of the rotation of Effect of the 

 the earth. By reason of the earth's rotation a body moving rotation 

 freely in the northern hemisphere in any direction will 

 be deflected to the right, and with great velocities this de- 

 flection is quite considerable. There are many examples of 

 it : a swinging pendulum constantly turns ; the wind does not 

 blow straight towards a cyclonic area, but in a spiral direction, 

 bending to the right in the northern, and to the left in the 

 southern, hemisphere ; the effect of the earth's rotation is also 



