BY W. Al TULEY, “ESQ.) BOs, “FSR:G.S., ETC. 133 
Immediately after each measurement was made and recorded, 
readings were taken from five standard thermometers which 
were placed alongside of the tapes at equal distances, fifteen 
readings were therefore taken during the three measurements 
of each tape. The mean of these readings must approximate 
very closely to the true temperature. The thermometers used 
were very carefully compared with a standard which was pro- 
cured from the observatory in Sydney, and the index errors 
determined. It is not probable that the temperature adopted is 
more than one-fifth of a degree Fahrenheit in error, but even 
were it in error half a degree it would not affect the length of 
the base by more than 1-3 inches.* 
The principal advantage of the system adopted, apart from 
the greater speed with which the measurement could be carried 
out, was the means it provided for checking the work as it pro- 
ceeded. In consequence of the length of the tape its fluctuations 
as the temperature altered were quite appreciable. I recollect 
when working at one of the microscopes seeing the tape visibly 
contract as a cold blast of air entered the troughs. It was 
often observed that the tape was more sensitive than the ther- 
mometers, and we often had to wait until the latter had settled 
down to the temperature of the tape. The co-efficient for ex- 
* After the base line had been measured six thermometers were supplied 
from the Royal Observatory, Kew, England, in response to the following 
special application made to G. M. Whipple, Esq., Superintendent :— 
“© QUEENSLAND. 
“ SURVEYOR-GENERAL’S OFFICE, 
“ BRISBANE, 22nd Jan., 1884. 
“Sir,—I trust I am not trespassing on your kindness in asking you to 
assist me in procuring a set of thermometers for the use of this depart- 
ment, in connection with the measurement of base lines in the field. 
**T am anxious to have 12 thermometers sent to me of a class suitable 
for recording the temperature in such measurements where great accu- 
racy is required. It is found, by experience, in sudden changes of 
temperature, that the steel tapes are more sensitive than ordinary 
standard glass thermometers; and it is possible that some modification 
