Of what extent our opening may be, is uncertain; but since captain Cook had smooth water in running to the west and northward to Cape Tribulation, where he first saw the reefs, it should seem to be not very great; certainly, as I think, not exceeding twenty, and perhaps not five leagues.  I therefore assume it as a great probability, that with the exception of this, and perhaps several small openings, our Barrier Reefs are connected with the Labyrinth of captain Cook; and that they reach to Torres’ Strait and to New Guinea, in 9 deg. south; or through 14 deg. of latitude and 9 deg. of longitude; which is not to be equalled in any other known part of the world.

The breadth of the barrier seems to be about fifteen leagues in its southern part, but diminishes to the northward; for at the Northumberland Islands it is twelve, and near our opening the breadth is not more than seven or eight leagues.  The reefs seen in latitude 173/4 deg., after we got through, being forty leagues from the coast, I consider to be distinct banks out at sea; as I do those discovered by Mons. de Bougainville in 151/2 deg., which lie still further off.  So far northward as I explored the Barrier Reefs, they are unconnected with the land; and continue so to latitude 16 deg.; for, as before said, captain Cook saw none until he had passed Cape Tribulation.

An arm of the sea is inclosed between the barrier and the coast, which is at first twenty-five or thirty leagues wide; but is contracted to twenty, abreast of Broad Sound, and to nine leagues at Cape Gloucester; from whence it seems to go on diminishing, till, a little beyond Cape Tribulation, reefs are found close to the shore.  Numerous islands lie scattered in this inclosed space; but so far as we are acquainted, there are no other coral banks in it than those by which some of the islands are surrounded; so that being sheltered from the deep waves of the ocean, it is particularly well adapted to the purposes of a coasting trade.  The reader will be struck with the analogy which this arm of the sea presents to one in nearly the same latitude of the northern hemisphere.  The Gulph of Florida is formed by the coast of America on the west, and by a great mass of islands and shoals on the east; which shoals are also of coral.

On the outside of the barrier, the sea appears to be generally unfathomable; but within, and amongst the reefs, there are soundings every where.  Nor is the depth very unequal, where the bottom is sandy; but like the breadth of the reefs and the arm they inclose, it diminishes as we advance northward, from 60 to 48, to 35, and to 30 fathoms near our opening; and to 20 at Cape Tribulation.  The further to leeward, the shallower the water, seems to be a law amongst coral reefs.

There is some variation in the tide in different parts of the barrier, but the most general rise is about two fathoms; abreast of the Northumberland Islands, however, where the flood from the south-east seems to meet that from the northward, it is three fathoms, and perhaps more.  The time of high water there, and also at the eastern Cumberland Islands, is eleven hours after the moon’s passage; but it probably accelerates north-westward, to the opening, and then retards further on:  at Endeavour River, captain Cook found it to be high water an hour and a half earlier than is above given.