490 PROFESSOR CHRYSTAL 



period, regulated by means of a metronome, to generate uninodal, binodal, and trinodal 

 seiches easily visible to a large audience. 



This method of generating seiches probably does not correspond to anything 

 observable under ordinary circumstances in a lake ; # but the experiment is interesting 

 in view of the important discovery recently made by the Japanese observers, t that 

 the secondary oscillations in many of the bays on the coast of Japan are seiches, 

 having a node at the mouth and a loop at the bottom of the bay. These oscillations, 

 which are sometimes of considerable range, are apparently due to resonance with 

 comparatively inconspicuous undulations in the external oceanic swell, the periods 

 of which are equal to some of the natural periods of the bay. 



It was also possible, by means of a trough like that above described (length, 5 feet ; 

 breadth, 4 inches ; depth, 5 inches ; depth of water, 3 inches), to illustrate the generation 

 of seiches in an ordinary lake by periodic variations of the surface pressure. By laying 

 a sheet of tin on the top of the trough, an air-channel was formed over the surface of 

 the water. Through this channel air could be blown by means of a Blackman's fan, 

 and, by working a slider timed by the metronome, the air-current could be made inter- 

 mittent. When the whole of the surface was covered over by the sheet of tin, the 

 effect of the current, whether steady or intermittent, was merely to generate a train 

 of progressive surface waves, the motion due to which was shown, by dropping in a 

 stream of red ink, to be confined to a stratum of the water near the surface. But, 

 when only half the length of the miniature lake was covered in, an intermittent current 

 having the proper period generated a uninodal seiche. When a strip of tin dipping 

 into the water at the end of the covering sheet just over the middle of the water was 

 used to block the air-current, after a few alterations of the blast the amplitude of 

 the generated seiche was such as to cause the water to splash over the ends of the 

 trough. 



In like manner, by covering in the tank up to the theoretical position of the binode, 

 a binodal seiche was generated, the parabolic surface of which at its culmination had 

 about the same curvature as the parabolic bottom of the trough. 



By using streams of red ink from a pipette, it was easy to demonstrate to the 

 audience the essential nature of the seiche-motion, and to contrast it with the essenti- 

 ally different motions which characterise the progressive and the solitary wave. 



On the Vibrations which cause the Embroidery on the Limnogram. 



To the oscillations of a lake-surface having a period of less than 2 m , which under 

 certain circumstances cause a regular or irregular embroidery on the limnogram, Forel 

 gave the name of vibrations. The complete explanation of these vibrations can hardly 



* Endros, however, has given examples in point, in some cases of constricted lakes, where a seiche in one part 

 forces a seiche of the same period in another part. 



t "Secondary Undulations of Oceanic Tides," by Honda, Terada, Yoshida, and Isitani, Joum. of the College 

 of Science, Imperial University, Tolcio, vol. xxiv., p. 1 (1908). 



