Page:A Voyage to Terra Australis Volume 2.djvu/345

Wreck Reef.]

with the Port Jackson rate, or 14′ 35″,9 less than the lunar observations. In laying down the Porpoise's track on the chart, this error has been corrected by an equal proportion, according to the time of each observation for the longitude.

Mr. Flinders deduced the variation of the compass from observing the sun's magnetic azimuth a.m. and p.m., when equal altitudes were taken, and comparing the mean azimuth at corresponding altitudes with the true meridian; this method is probably not the best, and the results from two compasses differed considerably; Walker's compass, marked No. 1, giving 9° 17′ east from ten observations, and that marked No. 1, 13° 54′ from five observations. The first is undoubtedly the best, though possibly not very correct.

There are here two regular tides daily, and it was high water on the day of full moon at 8ʰ 50′ in the morning; the rise was six feet two inches, but the night tide will probably reach to eight, or perhaps nine feet at the height of the springs.

Some account was given of Wreck-Reef Bank before quitting it in the boat, but I had not then acquired a knowledge of the whole extent of the reef. It is about twenty miles long, and from a quarter, to one mile and a half in breadth; and consists of many distinct patches of different magnitudes, the six principal of which are from four to eight or ten miles in circuit. They are separated by channels of one mile to near a league in width; and in the two easternmost I found from 8 to 10 fathoms, and nothing to prevent a ship passing through in a case of necessity. Four of the six larger patches have each a sand bank near the middle, which do not appear to have been lately covered by the tide; and they are now more or less frequented by sea birds, such as noddies, boobies, tropic, and man-of-war birds, gannets, and perhaps some others. Of these four banks, two lie to the west and one to the east of that near which our ships struck; but the eastern bank is the most considerable, and most frequented by birds; turtle also land there occasionally, and this bank was not improperly called Bird Islet, being now covered with coarse grass,