PREFACE. 



J. he present investigation of "Dead-water" was occasioned by a letter in 

 November 1898 from Prof. Nansen asking my opinion on the subject. In my 

 reply to Prof. Nansen I remarked that in the case of a layer of fresh water 

 resting on the top of salt water, a ship will not only produce the ordinary 

 visible waves at the boundary between the water and the air, but will also 

 generate invisible waves in the salt-water fresh- water boundary below; I 

 suggested that the great resistance experienced by the ship was due to the 

 work done in generating these invisible waves. In addition to some inaccur- 

 acies — at that time I was not intimately acquainted with the theory of water 

 wav es — my letter further suggested a plausible explanation of the suddeness 

 with which the resistance ceased: when the ship's speed has become so great 

 that the waves in the boundary below cannot follow her, the work done in 

 sustaining them will be saved. 



The matter was subsequently discussed once or twice when Prof. Nansen 

 and I occasionally met, and it was finally decided that I should have the sub- 

 ject rigorously examined by experiment. In December 1899 I consequently 

 suggested to a pupil of mine at the Stockholm's Hogskola, Dr. V. Walfrid Ek- 

 man, now assistant in the Central Laboratory for the International Study of the 

 Sea, Kristiania, that he should do some simple preliminary experiments. From 

 the very beginning they were successful and furnished results confirmatory of 

 the above mentioned assumptions. 



