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



indeed hardly to be expected, and all the more, because the observations were 

 made casually, and, in most cases, without any intention of afterwards giving an 

 accurate description of them. Nevertheless, as far as their main features are con- 

 cerned, they may all be explained from the boundary-waves which have been 

 studied above, theoretically and experimentally. Wind, currents, and other 

 circumstances vary in many ways, and may, as will be explained below, also 

 account for variations in the appearance of the phenomenon. 



Dr. Nansen observed, when he was steering in dead-water off Taimur, 

 very low waves following on the sea-surface, to the sides of the vessel and 

 in her wake (see Fig. 1, PI. IV). These waves are evidently the low surface- 

 waves, (see pp. 42 and 65 — 66), which are a secondary effect of the waves in 

 the boundary between salt and fresh water. Their crest-lines should then follow 

 the lines of the boundary-waves ; and indeed, it is not difficult to recognize the 

 shape of these on the sketch, Fig. 1, PI. IV. (See further below). The height 

 of the waves is also not in contradiction to this explanation. The photo- 

 graphs, Figs. 2 — 3, PI. XV and XVI — illustrating cases, which must have 

 been similar to that off Taimur — show boundary-waves of 2 or 3 millimetres' 

 height; or if allowance be made for the exaggeration of the wave-height on 

 account of the oblique reflection of the waves against the walls of the tank, 

 1*5 or 2 mm. at least. As the vessel on these photographs, is on the scale 

 1 : 1 000, it is very probable, that the Fram may have been followed by 

 boundary -waves of 1*5 or 2 metres' height from hollow to crest. The difference 

 of specific gravity between the two water-layers was supposed to have been 

 about 002, and the height of the surface-waves would then, according to for- 

 mula (2) p. 42, be nearly 0'02 of the boundary-waves, that is 3 or 4 cm. I 

 have observed that waves of such heights and of 10 metres' length or even 

 more, may be perceived without any difficulty, in smooth water. 



It is not very extraordinary, that the above mentioned long and low waves 

 have not been observed on any other occasion. Indeed, they will probably 

 escape one's attention it the sea is even moderately rippled by the wind ; and 

 in a very slight wind, the dead-water waves created will, as far as sailing- 

 vessels are concerned, be too low for them to be observed in the water-surface. 



Several narrators have seen two or more stripes on the sea-surface, 

 stretching obliquely abaft of the vessel or across her wake. They are de- 

 scribed in different ways — as streaks of hopping wavelets (accounts Nos. 3 



