TRIGONOMETRICAL SURVEY OF NEW SOUTH WALES. 179 
thus briefly described :—Each bar, encased in a box 6 inches by 
6 inches, a little less than 10 feet long, so that the ends project 
slightly, is supported on two camels having three distinct motions 
—yvertical, lateral, and longitudinal. At the leading end of each 
box there is fixed a microscope of peculiar construction, admirably 
adapted for the purpose, and possessing peculiar advantages over 
any similar appliance. (See engraving appended marked B. ) 
Its special feature is that it consists of two distinct object-g glasses 
fitted at the end of asingle tube, each glass having a wire in its 
focus viewed at the same time by an eye-piece common to both 
object-glasses, so that one of these wires, being brought by suit- 
able screws into exact coincidence with the spot at the front end 
of the bar to which it is attached, the rear end of the next bar 
can be brought into coincidence with the other wire; thus, in 
the process of measuring, the bars are never brought into actual 
contact, but are separated by the exact distance between the two 
wires of the microscope, which quantity had been determined 
with precision by very powerful micrometer microscopes at the 
Government Observatory. The great advantage of a microscope 
of this description is that, the lines of sight being parallel, the 
distance between two points in coincidence with the wires remains 
the same at whatever distance the points may be from the object- 
glasses. This contrivance was suggested by Mr. P. F. Adams, 
in order to prevent errors arising from change of focus in the 
microscopes formerly used on the Lake George base-line, and it 
has proved thoroughly satisfactory. 
In the actual measurement with the wooden bars, their lengths 
were uniformly determined before and after each day’s work 
by comparison with the standard, and the mean of these two 
determinations was used as the length of the bar for the day. 
The ground having been cleared and roughly levelled, each bar in 
succession was then levelled, aligned, and brought into position 
with reference to the proper wire of the microscopes, until the 
three were laid, after which the first one laid was carried to the 
front and so on till the end of the day’s work, when a point was 
firmly placed in the ground immediately below the terminal point 
of the foremost bar, accuracy of position of this point being secured 
by suitable apparatus. During the whole process the bars were 
protected from the direct rays of the sun by a succession of seven 
frames running on wheels and covered with canvas, so as to form 
one long tent, which was pushed along as the work proceeded. 
The measurement with the steel rods was made in almost 
exactly the same manner, the only difference being that the 
lengths of these rods were deduced by computation from their 
recorded temperatures when in use, instead of, as with the wooden 
bars, assuming that the mean between the morning and evening 
lengths would be equal to the mean length for the day. The steel 
