Secondary Undulations of Oceanic Tides. 117 
The tide-gauge nearest Krakatoa at the time of the 
eruption was that of Batavia. It recorded two-hour waves, 
but not a trace of one-hour waves appeared. The absence 
of these latter, however, raises no serious objection against 
our supposition, because the narrow opening to the north-east 
of the strait is very unfavourable for the propagation of the 
energy of the lateral oscillation, much more so than for that 
of the longitudinal. 
Thus the major part of the energy of oscillations was pro- 
pagated into the Indian Ocean, and left its record on the 
tide-gauges even so far distant as the ports of Southern Africa. 
Examining the records given in the above cited report, we 
may in general distinguish two types of undulations — the 
one those propagated directly from Krakatoa, and the other 
those of the stationary oscillation of bays or estuaries excited 
by the incident waves. Prominent undulations recorded 
along the coast of India belong to the former type. Com- 
paring the records at Madras and Vizagapatam or Negapatam 
and Port Blair, an identity of waves may easily be recognized. 
We see also the trace of Vizagapatam waves in Nagapatam 
records, and vice versa. Most of these stations are not situated 
in either a bay or an estuary where an oscillation of such a long 
period is possible. 
For remoter stations, we see in general that the disturb- 
ances are chiefly due to the second type, i. e., the proper 
oscillation excited by the synchronizing components of the 
incident waves. Hence for such ba} 7 s the periods of oscillation 
for Krakatoa waves must coincide with those calculated from 
the dimensions of the bays. To verify this point we require 
reliable charts giving all necessary data ; but having no such 
charts at hand, we had to be satisfied with deducing the mean 
depths of the different bays, quoted in the Krakatoa Report, 
from their periods of oscillation and their lengths estimated 
from charts * available. The results of our calculation for 
Port Elizabeth. Table Bay, Port Adelaide, Port Phillip, 
Lyttleton, and Honolulu all gave reasonably mean depths, 
when judged from the charts at hand. 
(ii.) Sea-waves accompanying Earthquakes. 
A number of great sea- waves caused by earthquakes have 
been recorded by the tide-gauges at different stations. We 
have studied these records, and found that the periods of the 
waves as given by the tide-gauge at a distant station are the 
* Berghaus, Physikalischer Atlas, Skitter's Atlas, and Encyclopedia 
Britannica. 
