100 EKMAN. ON DEAD-WATER. [norw. pol. EXP. 



given in an appendix below, that the transverse waves, very small at lower 

 velocities, increase forcibly by accumulation, when the vessel's velocity ap- 

 proaches the maximum wave-velocity. The reason of this is that much the 

 greater part of their energy is then transmitted with the same speed as the 

 waves and the vessel herself. The diverging waves will not be exaggerated 

 to a similar degree, at velocities approaching the above-mentioned critical 

 velocity, because even at this velocity, a fraction only of their energy, is 

 transmitted in the direction of the vessel's motion (see p. 37). The transverse 

 stripes, generated by the transverse waves, may therefore at these compara- 

 tively high velocities be more distinctly seen than the diverging stripes. At 

 lower velocities the diverging waves are more comparable in height with the 

 transverse waves, and it is obvious that in this case particularly the headmost 

 couple of diverging stripes should appear more distinctly than the other stripes; 

 because the water inside it. is owing to the boundary-waves, a mass of 

 oscillating currents which prevent the wind-ripples forming, and the headmost 

 stripes form the boundary between this unrippled, whirling area, and the area 

 outside it, which is rippled by the breeze (compare the description in the last 

 paragraph but one, on p. 20, as well as Fig. 1, PI. V). Quite in accordance 

 with this explanation, is the observation of Mr. Eriksen (Account No. 8) 

 that a couple of diverging stripes are seen when the wind is very slight, 

 but transverse stripes when the wind freshens a little. Some details in account 

 No. 7 (at the bottom of p. 16) probably point to the same facts as these ob- 

 servations of Mr. Eriksen. 



That the dead-water stripes really indicate waves in the boundary be- 

 tween salt-water and the lighter surface-water, is directly proved by an obser- 

 vation of Mr. G. A. Larsen (p. 14): when a vessel in dead-water has passed a 

 moored vessel, the latter is pushed backwards and forwards, as she is reached by 

 the stripes, one after the other. This shows that the water is really oscillating, 

 and the wave-length would, in consequence of statements given on p. 14, be 

 equal to twice 1 the distance between two stripes. It cannot be a case of 

 ordinary surface-waves, because with this wave-length, they would have an 



1 If the wave-length were equal to twice the distance between two stripes, it might be 

 expected that each second stripe would have a different appearance to the others. 

 Mr. Larsen says nothing about this, and 1 am not able to decide in what respect the 

 description might possibly be incorrect, or its explanation incomplete. It is not 

 improbable that the apparent disagreement depends on the phenomenon being different 



