SECONDARY UNDULATIONS OF OCEANIC TIDES. 89 



chart on sufficient scales, we satisfy ourselves by deducing the 

 mean depths of different bays, quoted in the Krakatoa Report, 

 from their period of oscillations and their lengths estimated 

 from the charts* available. The results of calculation are as 

 follows : — 



(1) Port Ehzabeth, Cape Colony. PI. LIT, Fig. 1. 



The node of the Algoa Bay was taken from Cape Receife 

 to the coast of Alexandria opposite to the Bird Island ; taking 

 the length / = 25km. and the period 7^=74™, we found the mean 

 depth h to be 52 m. 



(2) Table Bay, do. PI. LII, Fig. 2. 



The node was taken from Green Promontory to the coast 

 opposite to Rabben Island; taking ^= 11.3 km. and 7'= 60"", we 

 get li = lQ m. 



It will be noticed that the shape of the above two bays is 

 very similar to that of Hakodate, and also that the records of 

 secondary undulations in these bays are, in their character, quite 

 similar to each other. 



(3) Port Adelaide, Australia. PI. LII, Fig. 3. 



The period of 150'" traceable in the record of Port Adelaide 

 may be explained, if we suppose it due to the oscillation 

 between Port Adelaide and York Peninsula. In this case we 

 obtain // = 21.5ra. which very nearly coincides with the result of 

 our estimation from the chart. Besides, a period of about 200" 

 is traced, which is probably due to the oscillation of the In- 

 vestigator Strait, with nodal lines at its two ends. The period 

 calculated for the latter modes is 228"". 



(4) Port Phillip. PI. LII, Fig. 4. 



*) Berghftiis' Physikalische Atlas, Stieler's Hand Atlas, Encyclopedia Britanica etc. 



