36 ZEILON, ON TIDAL BOUNDARY-WAVES. 



separated by a thin black line of mixed water; the upper boundary of that stratum will 

 continue across the ridge into the left compartment. The hydrographic prototype is 

 the fjord, containing chiefly brackish water and above that a top-layer of fresh-water, due 

 for instance to the outflow of a river; the whole partly communicating with a salt-water 

 ocean. 



C. General Resnlts of the Experiments. 



A glance at any of the photographs of the plates at once shows that the qualitative 

 results of our theory are fully confirmed. 



There are always boundary-waves generated at the ridge, and ihey travel outwards from 

 it. The disturbance in the upper surface is at the same time relatively small. 



The first type of experiment (Pl. I) naturally should permit a more close comparison 

 to the theory. Our calculus showed us that, unless a rather strong tidal current were used, 

 no very sensible waves could be found. In the actual experiment the brass lamel moved 



some 6 cm in 3 seconds, consequently with a mean velocity of 2 c — , and to this corresponds 



SGC 



the maximum velocity cequalto 3,h , which is, in fact, about the order of magnitude 



of the boundary wave-velocity in that case. The waves ought then to show a non-uni- 

 form manner of propagation. This also was easily observed. When regarding for in- 

 stance the wave-system to the right, at a certain moment it would appear to you as stand- 

 ing quite still; then suddenly it would take a leap rightwards, then stop again, and so on, 

 in a manner obviously following the tide, as recorded by the ticks of the clock. 



As for the manner in which the waves are formed, it is seen that for each period 

 two waves, one on each side of the ridge, are generated. Moreover, much in accordance 

 with the drawings of fig. 2, pag. 21, the wave to the right begins to appear, and is developed 

 during that half of a period when the tide from running leftwards with decreasing strength 

 changes its direction, and gradually attains its maximum intensity rightwards. A very 

 close resemblance between the theoretical profiles and the observed ones is of course not 

 to be expected, especially as a strong tidal current certainly will influence rather much 

 upon the form of the wave in the vicinity of the barrier. 



It appears from Pl. I that, with the period and the difference of density there used, 

 the waves are sensibly simple-harmonic and of the fundamental period. It is easy, by 

 varying the experimental conditions, to obtain illustrations of the influence upon the 

 wave-form from the configuration of the ridge. For instance, if a strong salt-solution 

 (40 °/oo) was used for the bottom-layer, and the period of the pendulum enlargedto 

 about four seconds, now instead of two waves for each period jour were formed. 

 Thus the octave completely predominated, whereas no wave whatever of the fundamental 

 period could be observed. Certainly, since it is impossible to give the pendulum a simple- 

 harmonic motion of absolute purity, the effect may be complicated (in the manner indi- 

 cated on p. 30) by the octave already existing as a component of the original tide. 



