ising. The first thing was to determine the
direction of a line running due south from the topmost pinnacle of the
obelisk rock, and after a few trials with the compass, I got this. My
next act was to erect a line perpendicular to this along the sandy
margin of the basin, which I accomplished with the aid of my sextant,
taking care to make this second line as long as the nature of the ground
would allow. Then, driving a peg into the sand at the intersection of
these two lines, and another at the farther extremity of my second line,
I had a right-angled triangle, whereof the two pegs and the obelisk rock
marked the angles. I had now only to measure very carefully my second
line, which I did by means of a surveyor's tape measure, bought at
Sydney for the purpose, and to take the angle between the perpendicular
and the hypothenuse of my triangle, when I had the means of calculating
all or any of the elements of the triangle that I desired. In this way,
then, I ascertained that the pinnacle of the obelisk rock was exactly
six thousand four hundred and seventy-seven feet due north of the peg I
had driven into the sand to mark the intersection of my two lines.
Then, returning to this same peg, I sent Forbes away to the islet in the
boat, with instructions to set up one of the oars, with a white
pocket-handkerchief attached to it, on the shore of the islet at the
precise spot I should indicate to him by signal. This spot I arranged
to be exactly in line with the peg and the obelisk rock; all three
points, therefore, were in one straight line, the bearing of which was
due north and south, while its northern extremity was the obelisk rock.
My next task was to take an angle to the oar from the peg at which I had
taken the angle to the obelisk rock, which enabled me to determine that
the oar was three thousand eight hundred and two feet from the
intersecting peg, and consequently two thousand six hundred and
seventy-five feet from the obelisk rock. This completed all the data I
required; for I had now only to drive a bold, conspicuous staff into the
sand in place of my intersecting peg, and another into the ground on the
islet where the oar now stood, and by cutting back into the scrub for a
distance of one thousand six hundred and seventy-five feet toward the
obelisk rock, using these two staves as guides to keep their line
straight, the workers would reach a spot exactly a thousand feet south
from the obelisk rock; or, in o