34 ZEILON, ON TIDAL BOUNDARY-WAVES. 



of the intensity of the effects be the most interesting, and then no quantitative analogy 

 is attainable. Still, the fact that small-scale experiments, conducted under condi- 

 tions departing very far from those of the theory, show effects of the nature predicted 

 and increasing rapidly with the effectivity of the obstacle, might be taken as a very strong 

 evidence that the same may occur in the tidal case; and very probably the above law of 

 relativity may continue to be fairly true, when the theoretical basis of it has long since 

 ceased holding. But of course no conclusions can strictly be made, until experiments 

 are arranged on a tolerably true relative scale. 



B. Experimental Arrangements. 



The channel consisted of a glass tank, about 100 cm long and 3 cm broad. To ob- 

 tain a »tidal » wave-length as long as possible compared to the depth of the f luids, very 

 shallow layers of water were used, the total depth being on an average about 6 cm., and 

 ne ver exceeding 10 cm. The tidal wave was excited by means of the following pendulum 

 mechanism. A brass lamel was made to fit as near as possible into the cross-section of 

 the tank, its lower edge reaching very near the bottom. This lamel was mounted upon 

 a vertical shaft, some 80 cm long, turnin ground an axis at its other extremity, so that by 

 this means the lamel could be made to move along the channel for some 3 cm, with an 

 approximately rectilinear motion. A periodical oscillation was conducted to the lamel 

 from a heavy pendulum moving outside the tank. This pendulum was loaded on both 

 sides of its axis by lead we"ights; the upper one of which, if a changement of period was 

 desired, could be moved up and down. The whole system was worked by a strong clock- 

 work, and so adjusted that the heavy pendulum should determine, as far as possible, 

 the period of oscillation, and execute simple-harmonic vibrations, even if the resistance 

 was increased by the brass lamel dipping, at one end of the tank, into the water and push- 

 ing it to and fro. As a rule, a period of some 3 or 4 seconds was used, corresponding for 

 the total depth of 10 cm to a tide of a wave-length of about 3 or 4 meters. 



The arrangement, though working on the whole quite satisfactorily, had one draw- 

 back. The lamel, of course, could not fit quite tightly into the tank, so that the water 

 was allowed to pass between the lamel and the walls and the bottom of the tank, thus 

 causing disturbi ng vortices to be formed at the sharp edges of the lamel. The natural 

 effect was a sensible mixing up of the different layers of water, which would soon destroy 

 the beautiful appearance of the experiment. For this reason, the first moments after 

 the pendulum had been put in action were always chosen for photographing. 



The ridge of a maximum elevation of 4 cm was invariably fitted into the tank some 

 70 cm apart from the moving lamel. To avoid vortices, I gave it a somewhat broad and 

 rounded off shape. 



The best method of pouring the fresh-water above the salt-water, I found was to 

 let it impinge as a feeble jet upon a thin chip of cork swimming in the surface. Chinese 

 ink was always used for the colouring of one layer. The present experiments being only 

 of a qualitative nature, no care was taken to determine exactly the differences of density 



