256 
APPENDIX. 
[South Coast. 
ist. The instruments used in taking the distances, were a nine- 
inch sextant by Ramsden, and three sextants of eight inches radius 
by Troughton, the latter being made in 1801, expressly for the 
voyage. On board the ship, the sextant was necessarily held in the 
hand, and the distances were sometimes so taken on shore ; but in 
most of the latter cases, it was fixed on a stand admitting of the 
sextant being turned easily in any direction. The telescopes were 
of the largest magnifying powers which the motion of the ship, or 
state of the atmosphere could admit, and each longitude is the result 
of a set of observations, most generally consisting of six independent 
sights. They were taken either by lieutenant Flinders or by myself ; 
those by him being designated in the column of Observers by the 
letter F, the others by C. 
2nd. Preparatory to the reduction of the apparent to the true 
distance, the four following corrections have been applied. 
From the sun's semi-diameter, as given in the nautical almanack, 
3" have been subtracted. In the almanacks of the years compre- 
hending our observations, the semi-diameter was stated from Mayer's 
tables, which gave it 3" too great; owing to the imperfection of the 
telescope with which Mayer observed. 
The semi-diameters of the sun and moon being less in the 
vertical, than in the horizontal direction, on account of the differences 
in the refraction, they have been reduced proportionally to these 
differences and to the angles at the points of contact in measuring 
the distance. This correction is called contraction of the semi-diameter. 
Before using the moon's horizontal parallax in the nautical 
almanack, where it is calculated for the equator, it has been dimin- 
ished by a number of seconds depending upon the latitude of the 
place, and upon this assumed position : that the earth is a regular 
spheroid, whose polar axis is to the equatorial axis, as 320 to 321. 
This, and the preceding correction are unnecessary, unless where 
great exactness may be required. 
