The co-tidal lines, drawn at hourly intervals, show the tidal wave approaching the eastern shores of Australia from the Pacific, then travelling from east to west along the Southern Ocean, and from there flowing in a north-westerly direction across the Indian Ocean. It is obvious that in the making of such a map of co-tidal lines the imagination of the author has to be brought into play quite considerably, because we have no observations of the rise and fall of the water at points far out from land, and our actual observations, upon which the map of co-tidal lines is based, are confined to places on the shores of the continents and to islands.
Now there are no islands in the ocean to the south of Australia and it follows that the shape of the co-tidal lines in that region in this map must be determined by the progressive wave theory which the author has in his mind rather than by actual observation. There are serious difficulties to the acceptance of this simple theory of the progressive wave. Goldsbrough, for example, in a paper contributed to the Royal Society of London in , showed by mathematical calculation that, in an ocean extending from the South Pole to latitude 45 degrees or less, only quite small semi-diurnal tidal waves can he generated in such depths as are comparable with the Southern Ocean.
If however the Atlantic tides are derived from the Southern Ocean, the large semi-diurnal tides of the Atlantic clearly require that there should he large tides of a similar kind in the Southern Ocean. Moreover, although the tidal wave appears to travel in Atlantic from south to north it varies in height and speed in a way that is hard to understand if it is a simple progressive wave. It cannot be explained either by a variation in depth of the ocean or by a change in its width.
Again, going along the west coast of Australia from south to north, the tidal range at Springs from Cape Leeuwin up as far as Dirk Hartog Island is less than 3 feet but from there it increases rapidly until at Port Hedland it is 19 ft.
The progressive wave theory alone does not give us any reason able explanation of facts like these. Back to top 3. The Resonance Theory of the Tides There is another school of opinion that, at the beginning of the present century, was first firmly established by the work of Mr. Rollin A. According to Harris instead of looking for a progressive wave travelling right round the globe, we should rather consider the oceans as great basins of water which are continuously subjected to the disturbing effects of periodic tide-producing forces.
These may be divided out into forces of several different periods and the basin of water is capable of oscillating or setting up what is known as a stationary wave in many different ways. Out of all these many possible methods of oscillation there will probably be one or more that will keep time or nearly so with one or more of the tide-producing forces, which are continuously acting. If so, the water will naturally swing or oscillate in those ways which will keep time with the forces and those particular methods of oscillation will be emphasized and perhaps given a relative importance out of proportion to the forces which produce them.
It is the same principle as that of resonance. So, if the ocean is capable of oscillating in some way periodic, say, with the lunar forces, it will do so and the corresponding tidal forces will produce an effect greater in comparison with the effects produced by other tidal forces than we should expect from our knowledge of their magnitudes.
Thus the tide-producing forces due to the moon are about 2. There are places around the coast of Australia where the solar tide is just is as big lunar tide and other places where there is five or six or even, as on the New Zealand coast, ten times as big as the solar.
The most reasonable explanation of such effects that has been advanced is that they are due to the selective resonance of some adjoining body of water.
The repeated application of the tide-producing forces tests therefore to increase and emphasize the wave due to those forces that have this particular period. Harris accordingly made the attempt to divide the oceans of the earth into areas which he calculated, from his knowledge of their shapes and the recorded depths, would oscillate in synchronism with one of the components of the tide-producing forces, and then he made a map of the co-tidal lines of the world, based of course as previous ones on actual observations of the tidal round the shores, but with an entirely different view point in the mind of the author, Harris' map, so far as it affects the oceans around Australia is reproduced in Figure 2, and it will be seen that the co-tidal lines, especially those to the south Australia, are altogether different in form to those in the map of Dr.
See, who had at the back of his mind the idea of the progressive wave. In figure 1 the co-tidal lines to the south of Australia run pretty well north east and south west; in figure 2 they run roughly east and west.
According to figure 1 the time of high water along the southern coast of Australia increases progressively as we go from east to west and it takes just over four hours for the tidal waves to move along the entire southern coast from Cape Howe to Cape Leeuwin, over about 35 degrees of longitude. On the other hand according to Figure 2, the tidal wave approaches the south coast of Australia from the south and it is high water at the west end of it at the same time as at the east end of it.
It surely ought to be possible, one would think, to say defiantly which of these two views is correct. The south coast of Australia appears, according to this, to be in the position of being able to give decisive evidence for one side or the other. It is not however quite so simple as it may seem to form a definite opinion because, at most of our ports where observations are systematically recorded, the tidal wave has taken a considerable time to reach there from the open ocean.
The tide, for example, takes over six hours to travel up the comparatively shallow water of Spencer Gulf. It takes over three hours to traverse the shoals and channels of Port Phillip Bay from the Heads to Williamstown.
Obviously we must consider only ports close to the open ocean to reach which the tidal wave has not been forced to move over long stretches of shallow water.
The pamphlet on "South Australian Tide Tables for " issued by the South Australian Harbours Board gives the times of high water on full and change days at a number of ports along the coast. Taking the most easterly and the most westerly of these, Port Macdonnell, close to Cape Northumberland, and Port Eyre, near the head of the Bight, the time of high water is given as practically the same at both places, Port Macdonnell being two minutes later than Port Eyre.
These ports are separated by about 8 degrees of longitude, so that according to the co-tidal lines of Figure 1 we should expect Port Eyre to be about one hour later than Port Macdonnell. Again the Tide Tables issued by the Victorian Ports and Harbours Authorities give a list of tidal differences with reference to Williamstown as a standard port for various ports both in Victoria and other States, and they give the time of high water at Springs at Port Macdonnell as about 27 minutes earlier than at Warrnambool which has over 2 degrees of longitude to the east of it.
The time given at Port Campbell, which lies still further to the east, is with in two minutes of that at Port Macdonnell. In the Manual of Tides by Rollin A. Harris, published by the United States Coast and Geodetic Survey, a table is complied giving the co-tidal hour for high water at spring tides, that is the number of lunar hours between the time of high water at the place and the last transit of the moon at Greenwich, for a large number of ports on all continents.
The following list is taken from the table, for places on the southern coast of Australia, in order running from east to west. According to the co-tidal map of Figure 1 there should be a gradual increase in the co-tidal hours in this list of about three hours from top to bottom, but nothing of the kind is shown. The evidence seems to be definitely against the theory of the tidal wave moving from east to west along the Southern Ocean to the south of Australia.
On the other hand it shows that the front of the tidal wave approaching the southern Ocean must be in a general way approximately parallel to the shore. The two co-tidal maps show that the northern coast of Australia is affected by tidal waves which approach it both from the Indian Ocean at the western end and from the Pacific on the east.
These two tides mingle in the waters to the north of Arnhem Land. In Van Dieman Gulf at rising tide a stream setting eastward enters the Gulf from the north through Dundas Strait where it meets a stream setting eastward which enters through Clarence Strait.
Yet along the north shore of Arnhem Land the flood stream is in the main towards the east. Back to top 4. Tidal Ranges round Australia The range of tide along the northern coast of Australia is much greater than along the southern coast.
Asaph Bay on Melville Island the range is 14 feet. At Port Darwin the mean spring range is increased to 24 feet but it is sometimes as much as 30 feet. Further along, at Wyndham, at the western end of the apex of the Cambridge Gulf, it is 23 feet, and further along still, at Collier Bay, and Kings Sound, where we have by far the biggest tides of Australia, the spring range is as much as 36 feet in Collier Bay with a mean spring range of 34 feet at Derby.
Going further west the spring range gradually diminishes until we get to North West Cape. It is 28 feet at Broome, 19 ft.
To the south of the Northwest cape the spring range of tide becomes very much less. It is only 6 feet at Maud Landing, just to the south of it, 5 feet at Carnarvon, and at Geraldton it is only 2ft. From here on, to the south as far as the Leeuwin and along the western end of the south coast as far as Eucla, the range is only 2ft.
Going further to the east along the south coast the range increases. It is 5ft. The range, of course, increases beyond this as the tidal wave moves up the gradually narrowing Spencer and St. Vincent Gulfs. Further along the ocean coast it is 5 feet at Port Macdonnell, but diminishes to 3 feet at Portland and Warrnambool, and then increases again, being 5 feet at Apollo Bay and 5ft. At Gabo Island the mean spring range is 6 feet and it stands at around about 6 feet all up the east coast as far as Wide Bay, at the southern end of Great Sandy Island off the Queensland coast.
It is 5 feet at Jervis Bay, 6 feet at Sydney Heads, diminishing to 5. From here on, going north, the range increases. It is 11 feet at the entrance of the Mary River, 12 ft.
The Australia Pilot , issued by the Admiralty says "In Broad Sound, the flood streams from northward and southward meet, thus producing the great range of tide here found".
Doubtless this is accentuated by the configuration of the bay and the shallowing water. From there on, going further north, it diminishes again being 16ft. Along the shores of Tasmania the highest tide is along the northern coast where the spring range is about 10 feet at Stanley, Devonport, and Port Dalrymple, and at Roden and Hummock Islands in the Furneaux group at the eastern end of the north coast.
At Hobart the mean spring range is 4 ft. Back to top 5. Variations due to local conditions A certain amount of this variation in the recorded heights of tides may be bought about by the narrowing and shallowing of the channel along which the tidal flood stream progresses.
If the tidal wave enters a gulf which gradually contracts in width and decreases in depth the energy of the wave is spread over a continually diminishing area and the height of the wave is increased. There is a very good example of this in the behaviour of the tidal wave as it proceeds up the Spencer and St. Vincent Gulfs in south Australia. As we have already seen the tidal waver reaches Port Macdonnell, near the Victorian border, and Port Eyre, near the head of the bight, at about the same time.
It takes three hours and twenty six minutes to traverse Investigator Straight and Backstairs Passages, separating Kangaroo Island from the mainland, to reach Rapid Bay, on the eastern side of the entrance to St. Vincent Gulf and a point near Sturt Bay on the other side. At Rapid Bay the mean spring range is about 6 feet. The wave at this stage starts to travel much faster in the deep water in the middle of the Gulf than it does in the comparatively shallow water at the sides, with the result that the wave front becomes more and more curved as it proceeds, being much more advanced at the centre that it is at each side.
The consequence is that by the time it reaches the head of the Gulf, having traversed a distance a little short of miles from the entrance, it is practically parallel to the coast line all the way round and high water reaches the Semaphore, which is on the shore of the Gulf alongside Port Adelaide, Black Point on the western side of the Gulf, and Port Wakefield at the head of the of the Gulf, at the same time, one hour and twenty minutes after Rapid Bay.
Moreover as the wave proceeds up the narrowing Gulf it increases in height, the spring range being 6 feet at Rapid Head, 8 ft. Similar phenomena take place in Spencer Gulf to an even more marked degree, for it is about double the length and contracts to a narrower width. The tidal wave takes six and a half hours to go from Thistle Island, at the mouth to Port Augusta, at the head and the spring range increases from 5 feet at Thistle Island to 12 feet at Port Augusta.
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