86 K. UONDA, T. TEßADA, Y. YOSHIDA, AND D. ISITANI. 



to explain the period of two hours by assuming that the sea 

 bottom was upheaved for about an hour. According to Pro- 

 fessor H. Nagaoka,* the earth is continuously vibrating with a 

 period 07™ of fundamental oscillation, and it is this vibration that 

 actually determined the periods of the Krakatoa waves. Our 

 theory differs from the above by not assuming the slow up and 

 down motion of sea bottom. Now, according to the results of 

 our observations, any portion of sea partly bound by land, is 

 capable of its own mode of stationary oscillation, which if pro- 

 perly excited, may last for some time, after the cause of the 

 excitement has receded. In this respect, the Sunda Strait (Top. 

 09) presents a highly suggestive form by its boundary. The 

 south-west end of the channel opens widely into the Indian 

 ocean, while the north east end is narrow and shallow com- 

 municating to the Java Sea. The strait, as a whole, may be 

 compared in its acoustical analogy with a conical open pipe, 

 provided that in the case of the hydrodynamical problem, both 

 ends are to be considered as the nodes of the wave profile. The 

 loop of the gravest mode of oscillation possible in such a 

 channel must lie nearly midw^ay but somewliat nearer the nar- 

 rower end. Hence the eruption of Krakatoa lying nearly at the 

 loop of this oscillation would be very favourable to excite the 

 natural stationary oscillation of the strait as a whole. The 

 initial disturbance, would soon settle into a regular oscillation 

 natural to the system ; this again would be propagated into the 

 external ocean as a train of regular waves, whose period of 

 oscillation is determined by that of the source. 



Taking the length of tlie strait as IGOkm., and its mean 

 depth as 183 m. (i=100 fathoms), we obtain from our formula 



*J Prof. H. Nagaoka, Proc. Tokyo Math. Phys. Soc. 4. p. 35, 1907 ; Nature, May 24, 1907. 



