KUNGL. SV. VET. AKADEMIKNS HANDLINGAR. HAND 47. iN:<> 4. 37 



On the other hand, a ridge very long compared to the wave-length of free boundary- 

 waves of the given period may be able practically to extinguish the whole wave. As a 

 fact, in an experiment made with a very small difference of density - falling below 

 0,oi — the total absence of any boundary-waves was noted, there being left only a quasi- 

 standing oscillation across the top of the ridge. 



D. The Second Experiment and its Relations to Hydrography. 



This experiment affords a striking illustration of the general principles of chapter 

 G, Part I. For each period a quantity of black water is seen to be sucked out across the 

 ridge, and to propagate itself as a wave rightwards with a pulsatory velocity (Pl. II ). 



Initially here a ridge in form of a thin vertical screen was used. The following ex- 

 perience may be mentioned. The waves then produced showed a characteristic and ob- 

 viously not simple-harmonic profile with sensibly sharpened crests (pointing downwards), 

 whereas with the broad and rounded hill subsequently used the waves were quite smooth. 

 This evidently furnishes another illustration of the extinguishing influence of a broad 

 ridge upon the higher harmomes. 



Fig. 5. 



The present experiment shows some secondary phenomena of conspicuous interest, 

 especially with regard to its hydrographic analogy. The above diagram gives a sche- 

 matic view of the high-water situation in the vicinity of the ridge (compare Pl. III: 1,2). 



The free surface has attained its maximum elevation, and during the last half 

 of the preceding period the tidal current has been running across the barrier into the basin. 

 It is evident that, if no boundary-waves were formed, the current during this interval 

 would chiefly have caused fresh-water to be transported leftwards across the barrier. 

 since the boundary surface would then be very slightly elevated, perhaps not even high 

 enough to allow any salt-water at all to enter with the current. Now, however, waves 

 are formed in such a manner that at high-water the boundary is lifted above the apex 

 of the barrier to a height far exceeding the amplitude of the surface-tide. Thence we 

 draw the following consequences: 



1. During the flood-tide period salt-water passes the barrier through an increasing 

 portion of the entrance of the basin; the sectional area of the salt-water current attains 

 its maximum at high-water. 



2. At high-water there will not be zero current-velocity in the neighbourhood of 

 the ridge. As is shown in the diagram: The heavy salt-water, arrived at the top of the barrier, 

 will there be exposed to a gravity field of force, which will cause it to continue running for some 



