The botanists landed upon the east shore, preferring the main land for their pursuits; and the launch was sent to haul the seine on that side, at a beach a little way up the Sound. I went to the top of Pier Head and took bearings of the Northumberland Islands, as also of the points and hills of the coast to the east and west; the most essential of them to the connexion of the survey, were as under:
Mount Westall, station on the top, S. 63° 20' E.
Aken's Island, station on the N. E. end, S. 43 10 E.
Pine Mount, S. 25 5 E.
Long Island, the north point, distant 8 miles, N. 65 5 W.
Peaked Hill, west side of Broad Sound, N. 61 25 W.
Northumberland I., a peak, marked h, N. 22 25 W.
Northumberland I., No. 3 peak (of Percy Isles), N. 20 10 E.
Captain Cook observed, when taking bearings upon the top of Pier Head, "that the needle differed very considerably in its position, even to thirty degrees, in some places more, in others less; and once he found it differ from itself no less than two points in the distance of fourteen feet." (Hawkesworth, III, 126); from whence he concluded there was iron ore in the hills. I determined, in consequence, to make more particular observations, both with the theodolite and dipping needle; and shall briefly state the results obtained on this, and on the following day.
Azimuths were taken, and the bearing of Mount Westall, distant thirty-four miles, was set at S. 63° 28' E. (true), whilst the theodolite remained in the same place; and from a comparison between this bearing and those of the same object at different parts of the head, the variations were deduced. The dip was observed with both ends of the needle, and the face of the instrument changed each time.
At the highest top of Pier Head, Var. 3° 25' E. Dip 53° 20' S.
West, three yards from it, 6 10
S. E. three yards, 10 5
S. S. E., ten yards, 8 6 52 19
North, four, 6 55
N. E., twenty, 6 50 50 35
N. N. E., one-sixth mile, at the water side, 7 6 50 28
S. E., one-third mile, at ditto, 8 2 50 50
There are here no differences equal to those found by captain Cook; but it is to be observed, that he used a ship's azimuth compass, probably not raised further from the ground than to be placed on a stone, whereas my theodolite stood upon legs, more than four feet high. The dipping needle was raised about two feet; and by its greater inclination at the top of the hill, shows the principal attraction to have been not far from thence. The least dip, 50° 28', taken at the shore on the north side of the head, was doubtless the least affected; but it appears to have been half a degree too much, for at Port Bowen, twenty-two miles further south, it was no more than 50° 20'. An amplitude taken on board the ship in the Sound by lieutenant Flinders, when the head was S. S. W., gave variation 8° 39', or corrected to the meridian, 7° 40' east. As Pier Head lay almost exactly in the meridian, from the ship, its magnetism would not alter the direction of the needle; and I therefore consider 7° 40' to be very nearly the true variation, when unaffected by local causes: in Port Bowen, it varied from 7° 40' to 8° 30' east.
Notwithstanding this very sensible effect upon the needle, both horizontally and vertically, I did not find, any more than captain Cook, that a piece of the stone applied to the theodolite drew the needle at all out of its direction; nevertheless I am induced to think, that the attraction was rather dispersed throughout the mass of stone composing Pier Head, than that any mine of iron ore exists in it. The stone is a porphyry of a dark, blueish colour.
MONDAY 6 SEPTEMBER 1802
On the 6th, at noon, when the observations were finished and I had proposed to quit Thirsty Sound, the wind and tide were both against us. To employ the rest of the day usefully, I went over in the whale boat, accompanied by the landscape painter, to the 6th, 7th, and 8th Northumberland Islands, which, with many low islets and rocks near them, form a cluster three or four leagues to the north-east of the Sound. Orders were left with lieutenant Fowler to get the ship under way as early as possible on the following morning, and come out to meet us.
Nearly mid-way between Pier Head and the cluster, lie some rocks surrounded with breakers; and until they were passed the depth was from 6 to 8 fathoms, and 11 afterwards. We rowed to a beach at the north-west end of the 7th island, proposing there to pass the night, and hoped to turn some turtle; but proofs of natives having lately visited, or being perhaps then on the island, damped our prospects, and still more did the absence of turtle tracks; yet under each tree near the shore were the remains of a turtle feast.