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



While the "relief-photographs" (PI. XIII) chiefly establish the diverging 

 waves, it might be supposed that the waves shown by the "silhouette-photo- 

 graphs" (PI. XIV — XVII) are only the transverse waves. Actually, however, 

 diverging waves would look like transverse waves on these photographs: 

 aft of the launch Fig. 4, PI. XII, for instance, three sharply marked diverg- 

 ing waves are seen, and it is obvious that if the launch be regarded right 

 from the side, they should look like the section of a train of transverse 

 waves. A similar illusion will in a still greater degree take place in the glass- 

 tank; for the diverging waves become reflected against the walls of the tank 

 and, in consequence, have their height there increased. Further it is a con- 

 sequence of the total reflection of light in the boundary between salt and 

 fresh water, that every place where the dark-coloured salt-water only licks 

 the wall of the tank, appears quite dark, because the light reaching the ca- 

 mera from these places must have passed merely through salt-water from one 

 side of the tank to the other (supposing that the boundary between salt 

 and fresh water is smooth, and feebly inclined). On the other hand, where 

 the salt-water is raised in the middle of the tank, it will not appear quite 

 dark, because the water is there stirred by the vessel so that the light might 

 pass through it under diffuse refraction. The silhouette photographs must 

 therefore chiefly show the wave-profile along the walls of the tank and con- 

 sequently give the appearance of transverse waves. On the other hand, the 

 transverse waves must have really been comparatively more developed in just 

 those cases in which "silhouette-photographs" were taken, than when the relief- 

 photographs were taken. For in the former cases either a smaller tank (Pl.XIV) 

 or a larger boat-model (PI. XV — XVII) was used than in the latter cases; the 

 tank was consequently relatively narrower, and it is known that the waves 

 become in that case more quickly transformed into transverse waves. 



A photograph taken in June 1901 happened to show very clearly the true 

 shape of the waves which are shown by the silhouette-photographs. The 

 salt-water was rather strong (spec, gravity 1*16), but it was clear and not 

 coloured with Chinese ink. This photograph is reproduced in Fig. 1, PI. XII. 

 To make it more realistic, it is completed by a sketch of the upper part of 

 the Fram and her tackle. A relief-photograph of the small -FVam-model in 

 the small tank, is reproduced below (Fig. 2, PL XII), on such a scale that it 

 corresponds in all its details to Fig. 1. Figs. 1 and 2 examined together, give 



