Secondary Undulations of Oceanic Tides. 115 
approaching towards the shore, indicating that the waves are 
of: the nature of progressive, but not o£ stationary oscillations. 
When that disastrous wave of 1896 visited our coast of 
Sanriku, there were instances in which the periods of the 
wave did not coincide with those observed in ordinary 
cases. 
In the investigation of the nature of sea-waves, the tide- 
gauge is at present the sole instrument available ; and in 
order to obtain the best results it should be set up where 
there is an open coast, or better in the neighbourhood of a 
small isolated island. If the instrument be placed in a calm 
bay, as is usually done, the waves are much modified by the 
proper oscillation of the bay. 
In the following pages discussions relative to sea-waves, 
originated from three different causes, will be given. 
(i.) Sea-waves of the Krakatoa Eruption, 1883. 
Great sea-waves caused by the eruption of Krakatoa, 
August 27, 1883, swept over the entire area of the Indian 
Ocean, and even forced their way as far as the northern 
parts of the Atlantic and the Pacific, leaving as they went 
evidence of their visit on all intervening tide-gauges. These 
records have been published in the Report of the Royal 
Society of 1888, and described by Captain W. J. H. Wharton. 
As to the cause of the periods of these great Krakatoa 
waves, few theories have been proposed. Captain Wharton * 
attempted to explain the period of two hours by assuming 
that the sea-bottom was upheaved for about an hour. 
According to Professor H. Nagaokaf, the earth is continually 
vibrating to the period 67 m , which is the period of the funda- 
mental oscillation ; and this vibration was what actually 
determined the periods of the Krakatoa waves. Our theory 
differs from the above by not assuming the slow up and down 
motion of the sea-bottom. Now any portion of the sea partly 
bound by land, if suddenly excited by some great disturbing- 
cause, oscillates with its own stationary mode and may con- 
tinue for some time after the cause of the excitement has dis- 
appeared. Sunda Strait, from the character of its boundaries, 
ma}^ be taken as a good example. The south-west end of the 
channel opens widely into the Indian Ocean, while the north- 
east end, which is very narrow and shallow, leads to the Java 
Sea. The strait, as a whole, may be compared, by an acoustical 
analogy, to a conical open pipe. The loop of the gravest 
* Wharton ' The Report of the Krakatoa Eruption,' p. 97. 
t H. Nagaoka, Proc. Tokyo Math.-Phvs. Soc. iv. No. 2 (1907). 
12 
