BY W. A. TULLY, ESQ., B.A., F.R.G.S., ETC. 129 
aimed at in these comparisons. Hach division of the micro- 
meter microscopes used is about one-millionth of a yard. 
The determination of the temperature is of the utmost 
importance. The thermometers used require to be accurately 
graduated, and their index errors ascertained by careful com- 
parisons with a standard thermometer. The co-efficient for 
expansion is the same as that used for steel bars, and amounts 
to ‘(007632 inches in one hundred feet for one degree of tem- 
perature. This co-efficient is the one invariably adopted, and 
has been tested many times by the most careful experiments. 
The standard of our Queensland Survey is a bar of steel, float- 
ing in mercury, the length of which was determined by careful 
comparisons with the standard bars in the Sydney Observatory, 
which have been supplied by the Board of Trade in London. 
This bar has a length of 9°9998581 feet, at a temperature of 62° 
Fahrenheit ; so that, applying the above co-efficient for expansion 
of steel, its length at any other temperature can be determined. 
Having been provided with a standard, it was necessary to 
decide how the base should be measured. Having given the 
subject careful consideration, I determined to use a steel tape of 
one hundred feet in length, with such appliances as would 
ensure a high degree of accuracy. The experience subsequently 
gained in the measurement proves that I was right, as it is now 
clearly established that, with proper care, a base-line can be 
measured with equal accuracy with a one hundred-foot tape as 
with bars of shorter length. The next step was how to ascer- 
tain the length of the tape with reference to the bar. The 
necessity for doing this had not arisen before, as most of the 
base-lines had been measured with wooden or steel rods ten 
feet in length. I was fortunate enough in having Mr. Adams, 
the Surveyor-General of New South Wales, and Mr. Russell, 
the Government Astronomer of that Colony to assist me, and 
between them they devised an apparatus, by means of which a 
tape of one hundred or sixty-six feet can be compared with the 
