no. 16.] EXPLANATION OF THE DEAD-WATER PHENOMENON. 103 



belief that the vessel drags along a bulk of surface-water, is certainly an 

 easily explained illusion. 



As the boundary-waves follow the vessel, their wave-crests and wave- 

 hollows remain in an invariable position relative to the vessel. If the wave- 

 motion gives to the water at a particular spot a velocity with the vessel, it 

 would appear as though a bulk of water were being dragged along with her, 

 although it is really always a new mass of water which follows the vessel 

 for a short distance. It is exactly analogous to the case of a boat sailing 

 before the wind with just the same speed as a breaking wave at her side. 

 In the case of dead-water, on the other hand, the illusion will be more 

 complete, because the vessel moves at a slow velocity, and the waves causing 

 the motion of the water, are themselves not visible. 



It only remains to be seen in what direction the surface-water around 

 the vessel will be moved, as a result of the wave-motion. From the photo- 

 graphs PI. XV — XVII there appears to be a wave-hollow (elevation of the 

 boundary) right around the stern, and if this were the case, the surface-water 

 itself, would have a sternward motion. The photographs Figs. 2 and 3 PI. XIII 

 — and still better, Fig. 2 PI. XII — however, show that there is really a wave- 

 crest immediately around the stern, but not very far to the sides of it. The 

 surface-ivater nearest to the stern would consequently move in the same 

 direction as the vessel. Its velocity would not, according to the theory of 

 the boundary-waves, be so great as the velocity of the waves themselves 

 i. e. of the vessel (see p. 45); but it will be increased by the suction of the 

 vessel's stern, and the water might therefore follow the vessel. The situation 

 of the boundary-waves relative to the vessel, also agrees very well with the 

 statement (p. 13) that the water is pushed before her stem and is running 

 astern along her sides, at an increased speed. The fact that the wash of a 

 tug does not encounter the bow of a towed vessel, but is pushed aside (p. 14), 

 is obviously a consequence of the forward motion of the water before the bow. 



A few simple experiments were made in verification of the above conclu- 

 sions as to the motion around the stern. A small davit was fitted in such 

 a way to the stern of the boat (the large .Fram-model ; scale 1 : 100) that it 

 could be fixed in different positions. A small wooden bar was suspended 

 from the davit like a pendulum, and reached into the water; its inclination 

 then showed the velocity of the water relative to the vessel. In homogeneous 



