The mean is 7' 14" to the east of the lunars; but on using rates equally accelerated from those at Port Jackson to the above at Upper Head, and commencing the acceleration on Aug. 15, at Keppel Bay, where the time keepers were found to be keeping their former rates, the mean longitude will be 149° 48' 56.6", or 2' 3.6" from the lunar observations; which is therefore the presumable sum of their irregularities after August 15, or in 27.7 days.
In fixing the positions of places along the East Coast, I have made use of the time keepers from Port Jackson to Port Curtis, without any correction. From Port Curtis to Broad Sound, the coast and islands are laid down from theodolite bearings taken on shore, combined with the observed latitudes; and consequently the accuracy in longitude of the first portion depends upon that of Port Jackson and the time keepers, and of the last, upon Upper Head and the survey. These two unconnected longitudes meet at Port Curtis, and the difference between them is there no more than 5".
From observations with the theodolite upon the top of Upper Head, the variation was 8° 37' east; but on moving the instrument ten yards to the south-west, it was 45' less. At two other stations on the west side of the sound, it was 8° 15', and 8° 0'; and on board the ship 7° 17' and 7° 46', corrected. On the east side of the sound it differed at six stations on shore, from 8° to 6°; and on board the ship was 6° 44' corrected. As general results, therefore, but subject to many small deviations, the variation may be taken,
On the west side of Broad Sound at 8° 0' E.
On the east side 7 0
At the head of the sound it was,
at one station 12°, at another 10°;
the mean, 11 0
The differences between the two sides of the sound, both on shore and on board, are nearly similar to what took place in Shoal-water Bay.
The rise of tide and time of high water have been mentioned; but it may be proper to say what I conceive to be the cause of the extraordinary rise in Broad Sound. From Cape Howe, at the southern extremity of the East Coast, to Port Curtis at the edge of the tropic, the time of high water falls between seven and nine hours after the moon's passage, and the rise does not exceed nine feet; but from thence to the northward, commencing with Keppel Bay, the time becomes later, and the rise augments, till, at Broad Sound, they reach eleven hours, and between thirty and thirty-five feet. The principal flood tide upon the coast is supposed to come from the south-east, and the ebb from the north, or north-west; but from the particular formation of Keppel and Shoal-water Bays, and of Broad Sound, whose entrances face the north, or north-west, this ebb tide sets into them, and accumulates the water for some time, becoming to them a flood. This will, in some degree, account for the later time and greater rise of the tide; and is conformable to what captain Cook says upon the same subject (Hawkesworth, III. 244); but I think there is still a super-adding cause. At the distance of about thirty leagues to the N. N. W. from Break-sea Spit, commences a vast mass of reefs, which lie from twenty to thirty leagues from the coast, and extend past Broad Sound. These reefs, being mostly dry at low water, will impede the free access of the tide; and the greater proportion of it will come in between Break-sea Spit and the reefs, and be late in reaching the remoter parts; and if we suppose the reefs to terminate to the north, or north-west of the Sound, or that a large opening in them there exist, another flood tide will come from the northward, and meet the former; and the accumulation of water from this meeting, will cause an extraordinary rise in Broad Sound and the neighbouring bays, in the same manner as the meeting of the tides in the English and Irish Channels causes a great rise upon the north coast of France and the west coast of England.
That an opening exists in the reefs will hereafter appear; and captain Cook's observations prove, that for more than a degree to the north-west of Broad Sound, the flood came from the northward. I found, when at anchor off Keppel Bay, and again off Island Head, that the flood there came from the east or south-east; but when lying three miles out from Pier Head, there was no set whatever; and I am disposed to think that it is at the entrance of Broad Sound, where the two floods meet each other.
CHAPTER IV.
The Percy Isles: anchorage at No. 2.
Boat excursions.
Remarks on the Percy Isles; with nautical observations.
Coral reefs: courses amongst them during eleven days search for a passage through, to sea.
Description of a reef.
Anchorage at an eastern Cumberland Isle.
The Lady Nelson sent back to Port Jackson.
Continuation of coral reefs; and courses amongst them during three other days.
Cape Gloucester.
An opening discovered, and the reefs quitted.
General remarks on the Great Barrier; with some instruction relative to the opening.