38 



ZEILON, ON TIDAL BOUNDARY-WAVES. 



time down the lejt slope of the barrier into the basin (Pl. III, 1 — 2). At the same time, 

 as a compensation, there must, even though the effect be very feeble, be formed a surface 

 current of fresh-water, directed outwards from the basin. Thus, at high-water, there 

 is a tendency for a double-current system being established: a bottom-current of salt- 

 water into the basin and a fresh-water surf ace-current outwards into the ocean. 



The free surface now begins to fall, and the tidal current will for half a period be 

 running rightwards. Still, for some time, the bottom current will continue, whereas the 

 surface current is accelerated; and this state of things will last until the boundary above 

 the ridge has sunk so far that the heterogeneous gravitational effects are annihilated by 

 the tidal current. Through the whole entrance of the basin there will be a current running 

 outwards. The boundary will rapidly sink until it touches the apex of the ridge, and, 

 with decreasing strength of current, still farther until at low-water the configuration 

 shown in Fig. 6 is formed. 



Returning for a moment to the high-water situation, it is evident that the first mo- 

 ments of ebb-current should produce a very rapid fall of the boundary above the ridge, 

 since then necessarily the ebb-current to the right and the »gravitational » current to the 

 left will combine to that effect. The consequence is that during the ebb-period the en- 



Fig. 6. 



trance to the basin will be occupied by fresh-water for almost the whole time. The salt- 

 water introduced by the flood-current will thus be saved from being sucked out again by 

 the ebb, so that the total effect produced during a whole period will be: 



By the flood-current a certain quantity of salt-water is transported into the fjord, whereas 

 the ebb-current carries a corresponding amount of fresh-water out into the ocean. The periodic 

 recurrence of this pumping mechanism will then, as it were, throw the salt bottom-water 

 of the ocean in form of cascades across the barrier, behind which it will gather on the bottom 

 of the basin. It is easy to observe that the inclined position of the boundary to the right, 

 as indicated in fig. 4 of p. 35, is very favourable for the effectiveness of the operation. But 

 for the tide, the present hydrographical situation would be in equilibrium; the tide intro- 

 duced, the boundary-waves formed at the barrier will at once make the waters of fjord 

 and ocean circulate as a double-current of salt-water entering periodically and of fresh- 

 water running outwards with a pulsatory velocity. 



By observing particles of dust suspended in the water this conclusion may be 

 verified. 



The manner in which the salt-water spreads itself along the bottom of the basin 

 furnishes an interesting detail. Each separate cascade will be seen moving from the bar- 

 rier towards the inner basin end with a constant relative velocity against the prevailing 



