Myth of the Skip-holder. 303 



tainiiig a heavy bottom layer of salt water coloured with 

 India ink, having on top of it an uncoloured layer of" lighter 

 fresh water. Through this fresh-water layer he towed with 

 a eonstant or steadily increasing force a small boat model, 

 and studied and even photographed the boundary waves set 

 up in tiie fres;h-water salt-water boundary. He likewise 

 worked out the numerical results in a long series of extended 

 and complex mathematical equations, and as a result of his 

 experiments and calculations he states tliat : "It is proved 

 by the theoretical and experimental investigation above, that 

 a vessel moving in such a place creates waves in the boundary 

 between the two water layers, and, that on this account, 

 very marked effects on the speed of the vessel will occur ; 

 and it will be shown below^ that from the existence of such 

 waves all essential effects and peculiarities of the dead-water 



phenomenon can be vei-y ssimply explained it will, 



in addition, be shown that the resistance and speed reduction 

 due to the wave generation is of just the proper order of 

 magnitude to explain the effects of dead-water ; so that the 

 correctness of the explanation may Ije regarded as completely 

 substantiated '' '^. 



Fig. 9 (PI. XVII.) is copied from Ekman's photographs 

 showing how the retarding boundary wave is created and how 

 it affects the vessel. Of these photographs Ekman himself 

 writes : "The most important point, which the photographs 

 described above clearly show is that the waves largely in- 

 crease in height when the velocity of the boat increased 

 toward the critical velocity, but when this is passed, and the 

 boat is free from dead-water, the waves disajjpear.'^ In this 

 connection it should be noted that in (Ekman's) figures 

 A, B, C, the boat is in dead-water with boundary waves 

 steadily increasing in size. In ]), howevei', the velocity of 

 the boat has increased beyond the critical velocity and the 

 boundary waves have disappeared .... the boat is free from 

 or without dead-water. 



Fig. 10 (p. 301) is copied from Ekman from Scott-Russell 

 (a distinguished English engineer of the middle of the hist 

 century) to show the effects of towing a boat in shallow water. 

 Ekman uses it to explain the action of the boundary waves 

 in dead-water. " At the lower velocity, the boat pushes a 

 mass of water before her stem, and at her stern she provokes 

 a wave-hollow ; her resistance is in consequence increased 



* B. Helland-Haiiseu, in Sir John Murray and Dr. Johann Hjort's 

 •' The Depths of the Sea ' (1912). corroborated Ekman's conclusions, and, 

 calling this wave a " boundary wave," says that it " may stop a ship so 

 that it lies in dead-water hardlv able to move at all."' 



