62 EKMAN. ON DEAD-WATER. [norw. pol. exp. 



shown later, that the agreement between theory and experiment is in this 

 respect, really, very good. 



The laws of resistance will be more fully investigated, from a quantitative 

 point of view, in section F of this chapter. 



The waves generated by the vessel. 



The photographic illustrations will give a better idea of the waves in the 

 boundary, than any description will do; they must, however, be accompanied by 

 some critical remarks. The waves appeared quite distinctly even in the first 

 experiments on the small scale. Figs. 1 — 6, PI. XIV, show the aspect of these 

 waves as they were photographed from the side. — The salt-water, as well as 

 the boat, is seen quite black; the shallow fresh-water layer is brighter and al- 

 most exactly similar to the air, and between them the water-surface is seen 

 as a dark line; the boat is moving to the left. Figs. 2 and 3 are charac- 

 teristic of the shape of the waves when the boat is in "dead-water"; the salt- 

 water is then pressed down by the fresh-water in front of and under the stem 

 and is raised behind the stern — just as it should be according to the argu- 

 ments brought forward in Chapter II (p. 47). These disturbances of level give 

 rise to a train of waves following aft of the boat, which are even more di- 

 stinctly seen in the photographs taken from the large tank (Figs. 2 and 3, 

 PI. XV-XVII.). 



All these photographs only show silhouettes of the waves, and conse- 

 quently cannot give a complete idea of their shape. To get a better re- 

 presentation of the waves, some photographs were taken obliquely from 

 below, in the way described above (p. 58). One of these (Fig. 1, PI. XIII) 

 is projected in a frame representing the glass tank; and Figs. 2 and 3, 

 PI. XIII, may be imagined projected in the same frame. By help of Fig. 10, 

 PI. I, which shows how the photographs were taken, these latter can then 

 be understood without any difficulty. The greatest part of the imaginary 

 glass tank is filled with clear salt-water, and above this latter is a shallow 

 layer of fresh-water (dark in the figure). The effects of the refraction of light 

 in the water are not taken into account, as this would not make the figure 

 clearer. As the light falls from behind, each wave-crest in the fresh-water 

 layer is illumined on top and behind, while its fore-side, as well as each 

 wave-hollow, is shaded. Bearing this in mind, the shape of the waves can 



