Second Communication on the recent Earthquakes and Wave Phenomena.
[Read September 1, 1868.]
We have now evidence in addition to that which I formerly collected, that the first wave reached from the Chatham Islands to New Zealand in about an hour and a half, and that it passed on to the coast of Australia with slightly abated intensity in five hours more. This gives the average velocity at a little over 290 feet per second between the Chatham Islands and 360 feet per second from New Zealand to Australia, supposing that these places lie in the direct line of its progress.
From the ratio which this shows of increased velocity and diminished amplitude of the wave, a rough approximation of the distance from New Zealand, at which the wave originated, is obtained, and indicates it at over 3000 miles. If the earthshock which originated at the same time with the wave, had reached us, owing to its greater velocity, it would have travelled the same distance in less than half an hour, and be felt about ten hours before the sea wave was observed.
As compared with the speed at which oceanic waves have been discovered to cross the Northern Pacific, this velocity is much less, and may be due to the Southern Ocean having an inferior average depth.
I may remark that if efficient means were provided for the exact observation of such phenomena, we would be able to arrive at the solution of very important facts in physical geography, as from such waves we might determine the depth of the ocean; and from the earthquake shock we could arrive at the nature of the rocks which form its floor.
It is by the latter means alone that we can ever hope to learn anything of the geological structure of the great proportion, amounting to nearly three-fourths of the whole surface of the earth which is covered by the waters of the sea.
With respect to the direction in which the wave, we are considering, appears to have travelled, the reported circumstance of its reaching Sydney and Adelaide at the same time, while it does not seem to have been observed at Melbourne, most likely owing to the expanded form of the harbour, and the shelter afforded by Tasmania, where it was severely felt from the south-east, and lastly, its not having affected the inner parts of Auckland harbour, indicates that its course was from south of east.
I wish further especially to call attention to the rumours that there has been a recent rise of the land in this harbour, and that the tide does not now rise to its proper limits, on the other hand, that it falls lower than it formerly did, with the same state of the tides. This has been connected with the occurrence of the ocean waves, but it is to the slight earthquake shock which we experienced, that we must refer as the cause of the change in level, if it has really taken place. I have carefully examined many points of the shore of the harbour, but have failed to satisfy myself that a change has recently taken place; but the subject is one requiring further exact observations, extending over several periods of spring tides, before any conclusions can be arrived at.
Any elevation, however slight, occurring in such a mild mannar, will confer a great benefit to the country, when we reflect on the great advantages the Province derived from the previous elevation of land; as large tracts of the most valuable land were rendered available, which could not otherwise have been drained without a very large expenditure of capital.