104 _ THE GREAT TIDAL WAVE, MAY, 1877. 
_ It may be within the memory of some that between the 15th 
and 20th August, 1868, a succession of waves age Sydney, 
and were recorded by the self-r iepistarine tide- gauge. The 
average pearval between the waves was about 2 5 minutes, and the 
creates st oscillation 34 inches, measuring from t e crest of one 
wave to the hollow of the soe It was “thought at the time that 
they were earthquake phenom 
A similar visitation eachak 3 us agai ain in August, 1873, but it 
was not so marked in its character. The self-registering tide- 
gauge shows that the disturbance began during the afternoon of 
the 15th, and attained its maximum between 1 a.m. and 4 a.m. 0 
the 17th; the greatest oscillation, amounting to 5 inches, occurred 
between 3°15 a.m. and 3°33 a.m. of the 17th ; the average interval 
of the waves at ig time was = nee: ‘but the average of 20 
between 8 p.m. and 5°30a.m., was 28 minutes. The nore could 
not be traced beyen the 18th. (See ‘Na alure, Oct., 
apes wet rape situated about 60 miles oe Be Sydnay, 
tidal wave made its appearance on gauge (shown by a full 
ns on Secieun at 5°20 a.m. (same as 8 Sydney) on the 11th 
May, 1877. At the greatest oscillations the water fell 31 inches 
in about 10 minutes, after which the oscillations gradually died 
average interval between the waves is about 
is fixed in a well, the sides of which are formed of ballast; an 
ordinary wave washes in and out freely. This port has a 
- across the heads, which no meant helped to break the waves 
before they reached the gau 
Ballina, situated ase nah miles north of Sydney Heads. 
lace was also visited with a similar phenomenon ; it was 
observed all wo ; greatest rise, 18 inches. No gauge there ye 
obtain information 
Brisbane.—Several ie disturbances mre teens observed in 
Moreton Bay. No gauge there for informat 
IT have no records of flied e waves from the Soathers ee 
0. 
resemblance to the true form of the natural wave, sitheoek for 
engineering purposes, arguments, and petrenes, the outline in 
the diagram 1s correct. _ ses — the difference in the 
— and vertical seal 
On diagram T have hina: ae rise ans fall of she surface of ) 
“the harbour due to the tidal wave from the Pacific Ocean. 
Pigott: Ca esas 
