PERIODIC WAVES. 269 
there were many not explained. Then came the great earth- 
quake in New Zealand, on June 10, 1886, when the Pink Terraces 
were destroyed, but no periodic waves were recorded ; the earth- 
quakes in South Australia, August 22, 1896, and many others 
which left no trace on our tide-gauges, although in the intervening 
years many periodic waves were recorded. Captain Allan, the late 
harbour-master at Newcastle, who had charge of the tide-gauge, 
observed in a report to me “that when the tide-gauge sheet 
showed sharp-pointed, oscillating marks, a gale followed within 
forty-eight hours, accompanied with high tide and heavy sea.” 
He continued :—“T also have observed that, although the tide- 
sheet may show considerable oscillation of a rounded character, it 
is not always followed by a gale. It is only when the pencil 
shows sharp-pointed, zig-zag lines along the tide curve that a gale 
may be looked for, and the more defined and the greater the range 
of these zig-zag lines the heavier the coming gale.” The frequency 
of these periodic waves had long before led me to look amongst 
ordinary meteorological conditions for something which would 
give rise to them; and the daily weather-chart, giving a synoptic 
picture of all the weather conditions, made evident the possibility 
of the origination of an impulse such as would generate waves. 
About every seven days an anticyclone passes over Australia, 
and between one area of high pressure on the next one, there is 
always an area of low pressure called a A depression in which is 
the lowest barometer, and the sides of the A have the steepest 
grades, on the preceding side of the A are strong N. to N.E. winds, 
and on the following side strong 8. to S.W. winds. The change 
of winds is sudden and the southerly winds is one of the strongest 
we have. 
The barometer sometimes falls to 29:000. Under such a low 
pressure the sea rises and currents set in on all sides except the 
north, and there the mainland of Australia is found and cannot 
supply the demand for water, so that the currents into the A set 
in from the other sides. During this time the whole system is 
travelling to the east, accelerating the currents into the centre, 
until when it approaches Bass Straits all the easterly current is cut 
off by Tasmania, except that which comes through Bass Straits ; 
meantime the falling barometers have accelerated the constant 
southerly set of the east coast current, making it a very strong 
northerly current which meets the storm centre ; with its westerly 
and southerly currents in Bass Straits, a confined area, in which by 
the meeting of these currents, the waves are set in motion which 
we find recorded on the gauge. This southerly setting current is so 
vigorous that it continues to set to south in opposition to the 
strongly opposing southerly winds which meet it as the centre of the 
A depression passes. This fact is well known ; Captain Dawson 
of H.M.S. “ Waterwitch” informed me that it was well known 
