TRIGONOMETRICAL SURVEY OF NEW SOUTH WALES. 219 
determined in position (that is, observed at the largest number of 
different observatories, not the one all the observations of which, 
though numerous, may have all been made at only a few places), 
reducing its place and proper motion rigorously, and determining, 
by differences, the places of the principal other stars in the same 
subdivision, using the selected star as a point of reference. In 
this way the best information would be obtained for a large 
number of stars distributed generally over the sky, and the tabular 
positions of the remainder would be considerably improved with 
comparatively moderate labour. A very necessary part of such a 
work would be the discussion of the systematic errors of each 
observatory. 
AZIMUTH OBSERVATIONS. 
Astronomical azimuths have been usually found by observation 
of circumpolar stars at meridian transit. 
CONNECTION OF THE ASTRONOMICAL AND GEODETIC RESULTS. 
In a triangulation such as the one under review, extending from 
latitude 33° to latitude 37°, and covering some 3 degrees of 
longitude, comparison of the astronomically determined positions 
with those obtained through the terrestrial measurements afford a 
means of estimating the form of the surface covered by the survey, 
and, when combined with other similar works, of determining the 
dimensions and shape of the earth. The consideration of this 
subject is, however, very largely influenced by the existence of 
local attraction, by which the position of the apparent zenith at 
any place is liable to be deflected to the extent of several seconds 
of arc, owing to the irregular distribution of matter in the vicinity 
of the point at which the astronomical observations are made, and, 
even where the point may from its surroundings be apparently 
free from any suspicion of local attraction, the apparent zenith 
may be displaced. As evidence of this the following passage is 
quoted from one of our highest authorities,* who writes :—“ This 
amount indeed is often exceeded, and it is not uncommon to find, 
as in the vicinity of Edinburgh, a deflection of gravity to the 
extent of 5”, whilst in the counties of Banff and Elgin there are 
cases of still larger deflections, the maximum of 10” being found 
at the village of Portsoy. At the base of the Himalayas, where 
we should naturally expect a large attraction, it amounts to about 
30”, diminishing somewhat rapidly as the distance from the 
mountains increases. A very remarkable instance of such irregu- 
larities exists near Moscow, brought to light through the large 
number of observed latitudes in that district. Drawing a line 
* Geodesy, Colonel A. R. Clarke, Oxford, 1880, page 228. 
