TRIGONOMETRICAL SURVEY OF NEW SOUTH WALES. 181 
COMPARISON OF BASES. 
The combined errors of measurement of the two bases and of 
the intervening triangulation, produced an apparent discrepancy 
of only one and two thirds inches i in the length of the Lake George 
base. The bases were assumed to be correct and an adjustment 
of the triangles was made in order to eliminate this small, appar- 
ent difference. 
INSTRUMENTS. 
Azimuthal directions have been measured with 18-inch altazi- 
muth instruments made by Troughton and Simms, two of which 
have been employed. The one generally used is an excellent 
instrument with a 3-inch object-glass and filar micrometer eye- 
piece, with eye-piece motion to view any part of the field. <A 
level attachment is provided for the telescope to enable it to be 
used in observation of latitude by the Talcott method. The 
horizontal circle is firmly attached to the base of the instrument, 
the telescope and four equidistant micrometers, by which the 
circle is read, revolving upon its centre. Two lamps are used for 
illumination, and by a system of prisms and reflectors the parts of 
the horizontal circle immediately under the micrometers or the 
similar parts of the vertical circle may be read at will. The 
altazimuth is mounted on a steel cone, in place of the ordinary 
wooden stand, the base of the cone being usually embedded in 
cement upon a solid foundation, and the top of the cone furnished 
with a movable ring, by which the whole instrument may be 
revolved over the mark constituting the station. These marks 
are made in metal plugs set where possible in solid rock and 
otherwise in stone masses buried in the ground, the plug being 
surmounted by a masonry, concrete, or rubble cairn on which is 
erected a pile, usually of 4 in. x 4 in. pine, carrying metal vanes 
of 3 feet or more radius. 
With regard to instruments for work of this class reference 
must be made to the recent report by Dr. Gill on the geodetic 
survey of South Africa, which contains so much valuable matter 
that it may be said to, in a manner, mark an epoch in geodesy. 
In it mention is made of some points which will probably have an 
important influence on the future methods of conducting the field 
work of a triangulation, and not least among these is his discus- 
sion of the relative values of large and small theodolites. In this 
connection he states that during the course of the survey it was 
shown that the accidental errors of pointing or reading were less 
a be regarded than those caused by lateral refraction. Not only 
vas it found that a change of wind was sometimes followed by 
misksa change in the direction of a distant station, but that a 
systematic difference, small but appreciable, existed between 
