Currents and Tides upon the Coast

I shall conclude the account of this Country with a few observations on the Currents and Tides upon the Coast, because I have mentioned in the Course of this Journal that the latter hath sometimes set one way and sometimes another, which I shall Endeavour to account for in the best manner I can. From the Latitude of 32 degrees, or above downwards to Sandy Cape in the Latitude of 24 degrees 46 minutes, we constantly found a Current setting to the Southward at the rate of 10 or 15 Miles per Day, more or less, according to the distance we were from the land, for it runs stronger in shore than in the Offing. All this time I had not been able to satisfy myself whether the flood-tide came from the Southward, Eastward, or Northward, but judged it to come from the South-East; but the first time we anchor'd upon the coast, which was in the Latitude of 24 degrees 30 minutes, and about 10 Leagues to the South-East of Bustard Bay, we found there the flood to come from the North-West. On the Contrary, 30 Leagues further to the North-West, on the South side of Keppel Bay, we found the Flood to come from the East, and at the Northern part of the said Bay we found it come from the Northward, but with a much Slower Motion than the Easterly Tide. Again, on the East side of the Bay of Inlets we found the flood to set strong to the Westward as far as the Op'ning of Broad sound, but on the North side of that sound the flood come with a Slow motion from the North-West; and when at Anchor before Repulse bay we found the flood to come from the northward. We need only admit the flood tide to come from the East or South-East, and then all these seeming Contradictions will be found to be conformable to reason and experience. It is well known that where there are deep Inlets, large Creeks, etc., into low lands, that it is not occasioned by fresh water Rivers; there is a very great indraught of the Flood Tide, the direction of which will be determin'd according to the possition or direction of the Coast which forms the Entrance into such Inlets; and this direction the Tide must follow, let it be ever so contrary to their general Course out at Sea, and where the Tides are weak, as they are in general upon this Coast, a large Inlet will, if I may so call it, attract the Flood tide for many Leagues. Any one need only cast an Eye over the Chart to be made sencible of what I have advanced. To the Northward of Whitsundays Passage there are few or no large Inlets, and consequently the Flood sets to the Northward or North-West, according to the direction of the Coast, and Ebb the Contrary; but this is to be understood at a little distance from land, or where there is no Creeks or Inlets, for where such are, be they ever so small, they draw the flood from the Southward, Eastward, and Northward, and, as I found by experience, while we lay in Endeavour River.* Another thing I have observed upon the Tides which ought to be remarked, which is that there is only one high Tide in 24 Hours, and that is the night Tide. On the Spring Tides the difference between the perpendicular rise of the night and day Tides is not less than 3 feet, which is a great deal where the Tides are so inconsiderable, as they are here.**This inequality of the Tide I did not observe till we run ashore; perhaps it is much more so to the Northward than to the Southward. After we had got within the Reefs the second time we found the Tides more considerable than at any time before, except in the Bay of Inlets. It may be owing to the water being confin'd in Channels between the Shoals, but the flood always set to the North-West to the extremity of New Wales, from thence West and South-West into the India Seas.

* Cook's reasoning on the course of the flood stream is quite sound.

** This difference in the heights of consecutive tides is termed the diurnal inequality. It results from the tide wave being made up of a large number of undulations, some caused by the moon, some by the sun; some occurring twice a day, others only once. It occurs in all parts of the world, but is inconspicuous on the coasts of Europe. In Australia it is very marked, and occasions the night tides to be the highest at one time of the year, when the Endeavour was on the coast, and the day tides at the other. There are places on the east coast of Australia where the range of the tide is very great, but Cook did not anchor at any of them.