TERRA AUSTRALIS. 
55 
Thirsty Sound.] 
whereas my theodolite stood upon legs, more than folir feet high. The iso2- 
dipping needle was raised about two feet; and by its greater inclination Sunday 5. 
at the top of the hill, shows the principal attraction to have been not 
far from thence. The least dip, go° 28', taken at the shore on the 
north side of the head, was doubtless the least affected ; but it ap- 
pears 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 Flin- 
ders, 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 0 40' to be very nearly the 
true variation, when unaffected by local causes : in Port Bowen, it 
varied from 7 0 40' to 8° 30' east. 
Notwithstanding this very sensible effect upon the needle, 
both horizontally and vertically, I did not find, any more than cap- 
tain 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. 
On the 6th, at noon, when the observations were finished and Monday <5. 
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 morn- 
ing, 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 
