THE TIDES OF SOUTH AUSTRALTA. 243 
shallows at the sides, so that it is everywhere high water in the 
middle of the gulf some considerable time (about one and a 
quarter hour opposite the Semaphore) before it is high water 
at the opposite points on the shore at each side. By the 
time the wave has reached the head of St. Vincent Gulf, 
the co-tidal line is practically parallel with the coast line 
for some considerable distance all round, so that the wave 
reaches Port Wakefield, at the head of the gulf, at the same 
time as it reaches the Semaphore and Black Point at the 
sides, and the time of high water is the same at the three 
places. Further, as the wave travels up the gradually narrowing 
gulf into shallower water, its speed diminishes but its height 
increases ; so that the mean spring range of the water is 6 ft. at 
Rapid Head, 8 ft. 3 in. at Port Adelaide, whilst at the head of 
the gulf at Port Wakefield the range is 11 ft. Again, going up 
Spencer Gulf the spring range is 5 ft. at Thistle Island, but 
becomes increased to 12 ft. at Port Augusta. The tide enters 
St. Vincent Gulf by the two channels round Kangaroo Island, 
but unfortunately observations have not been taken at a sufficient 
number of points round the island to enable us to trace ont the 
co-tidal lines. It is peculiar that high water reaches Ante- 
Chamber Bay one and three-quarter hour before it reaches Cape 
Willoughby. Apparently when the tide enters the gulf there is 
a strong current setting across from Sturt Bay on the peninsula, 
to Hog Bay on the island, as has been evidenced on several 
occasions by the way in which wreckage has been carried. 
A peculiar tidal phenomenon takes place at Port Lincoln, which 
was noticed by Flinders. He observed there that “the tide did 
not exceed 34 feet, and that, as in Princess Royal Harbour, there 
was only one high water in twenty-four hours, which took place 
at night, about eleven hours after the moon’s passage over 
the meridian. Yet at Thorny Passage, which is but a few 
leagues distant, there were two sets daily. This difference 
in so short a space appears extraordinary; but it may 
perhaps be accounted for by the direction of the entrance 
to the port, which is open to the N.E., from whence the ebb 
comes.”* The suggestion here made by Flinders gives us 
the explanation of the phenomenon, when we take into 
account as well the diurnal inequality. Thus at Port Lincoln, 
the observable tide is really the higher one of the two daily tides. 
Owing to the direction of the outlet of the harbour the water 
cannot get out freely, as the harbour catches the ebbing tide from 
the gulf, with the result that the level of the water falls very 
slowly ; so slowly in fact, that the second lower tide which follows 
in the course of the day does not appreciably raise the level of the 
Captain Flinders, Terr, Aust., Vol. I, p. 150. 
