228 PROCEEDINGS OF SECTION A. . 
estimated that the adopted temperatures are probably not more 
than one-fifth of a degree in error. Marks were made in copper 
discs inserted in wooden pegs driven into the ground at each 100 
feet, the distances between the tape-ends and the marks being 
measured by micrometer microscopes. Three measurements were 
made with each tape, each one distinct and independent, so that 
every section was measured six times. For the whole base the 
difference between the means of the three measures with each 
tape amounted to 0-117 of an inch; a slightly greater difference 
than this was, however, observed in the corresponding measures 
of one of the sections—section 5. The lengths of the various 
sections were compared by triangulation, and calculating one- 
half of the base from the measurement, a difference of -936 of an 
inch was shown. The base-line work was executed by Mr. A. 
McDowall, the present Surveyor-General of Queensland, assisted 
by Mr. R. Hoggan. 
The instruments used in the angle measures were generally 
10-inch theodolites by Troughton and Simms, read by two 
micrometer microscopes to a second of are, but at a few of the 
stations a 12-inch instrument by the same makérs was employed. 
From two to eight readings were made on each of eleven different 
parts of the graduated arc, the mean of the means in - each 
position being used. The following are the closing errors of the 
seventy-four measured triangles :— 
Closing error. Number of triangles. 
0” bo: a? 29 
I? the 22? 29 
2” to: 3” 11 
Upwards of 3” 5 
The maximum error of close was 3:90, and, computing m from 
SACS ee ; : 
m =(= ye it is found to be (+) 0”°95. The triangulation has 
n 
been calculated with elements given by Colonel James in the 
account of the Ordnance Survey of Great Britain in 1858. 
The astronomical datum is the position of the station at 
Jimbour, as determined by Captain Morris, R.E., and Lieutenant 
Darwin, when observing the Transit of Venus in 1882, the 
longitude being measured by telegraphic exchange of time signals 
with Sydney. Astronomical observations have been made at 
stations, Bloodwood, Brisbane, Haystack, and Mount Domville. 
The geodetic latitudes of these stations, minus the astronomical 
positions, give the results —-02, +117, +5737, and — 117, 
respectively, and the similar differences for the longitudes of the 
first two of the stations named are +657 and —-05". The 
azimuth datum was observed by meridian transits of circumpolar 
stars at station Bloodwood, and the latitude of the same station 
