1889.] Geology and PalcEotttology. 497 
is not clear ; but if the tide gauges can be relied upon, and the 
disturbances recorded are due to identical original waves, it 
seems probable that submarine elevations and ridges, hitherto 
unknown, retard the progress of the disturbance. The period 
of the long wave was originally about two hours, but at distant 
stations, such as Orange Bay and the ports of the English 
Channel, the period seems to have been reduced to about one 
fourth, and, throughout the course of the undulations, its orig- 
inal character appears to have undergone considerable modifi- 
cation. The cause of an undulation with a period of two hours 
remains a mystery, but of the correspondence between the 
water and air waves in point of time at starting there can be no 
question. An upheaval of the sea bottom must have been very 
slow to account for the length of the wave ; no earthquake was 
observed, and the evidence generally is against earth disturb- 
ance as a cause. It is noted that the bulk of the fragments 
thrown out during the explosions must have fallen into the sea, 
and by their impact, almost coinciding with the violent evis- 
ceration of the crater, must have contributed to the rush of the 
destructive waves, and Captain Wharton calculates that a 
fiftieth part of the missing mass of Krakatoa, which was esti- 
mated to be at least 200,000,000,000 cubic feet, would, by 
dropping suddenly into the water, form a wave circle of lOO 
miles in circumference, 20 feet high, and 350 feet wide. But 
this is inadequate to account for the long wave ; and he there- 
fore holds that the destructive waves in the Strait of Sunda 
were mainly due to masses falling into the sea, or to sudden 
explosions under the sea, but that the long wave recorded by 
distant tide guages had its origin in upheaval of the bottom. 
Another marked accompaniment of the explosion was the 
air wave. Reports from 47 stations representing the entire 
civilized world show that an air wave spread out from Kraka- 
toa as a centre expanding in a circular form till half round the 
globe, concentrated again towards the Antipodes, whence it 
started afresh and travelled back to Krakatoa, occupying in 
the double journey 36 hours, rebounded and set off again on 
the same revolution, and repeated the movement af least three 
times sufl^ciently strongly to be recorded. Seven passages, 
going and returning were indicated by the diagrams at some 
stations. The whole process was almost exactly similar to the 
alternate expansions and contractions of a wave of water 
caused by dropping a stone at the centre of a circular pool. 
The barograms give tidings of atmospheric movements com- 
parable to gigantic waves of sound, starting from a small area 
