132 MEASUREMENT OF BASE LINE, 
was marked by a piece of sawn hardwood, 4 x 4 inches, and 
three feet long, strongly driven into the ground. On the top of 
the peg a small copper stud, with a flat dise (a copper rivet was 
used for the purpose) was inserted to receive the mark at the 
end of each hundred feet. Three measurements were made 
with each tape, each one distinct and independent, so that every 
section has been measured six times. A tension of 20 lbs., by 
means of a spring balance, was applied to each tape. The 
following extract from the instructions issued for the 
guidance of the surveyors employed in the measurement will 
supply the particulars in regard to the microscopes used. I 
have brought down similar ones for the inspection of members :— 
‘““Two microscopes are to be used—one at each end of the 
tape. The ‘following’ one is for the purpose of keeping the 
end of the tape and the mark previously made on the copper 
stud co-incident, whilst, by means of the ‘leading’ microscope, 
a mark is made on the copper stud, as nearly as possible agreeing _ 
with the terminal point of the steel tape. The ‘following’ 
microscope being set to zero, the ‘leading’ one is used for the 
purpose of comparing the mark made on the copper stud with 
the terminal point of the tape in each measurement, and regis- 
tering the micrometer value of the interval—if any—between 
them. The micrometer microscopes used have a value repre- 
sented by two revolutions of the screw to one hundredth of 
an inch; and with the view of lessening the risk of 
error by recording plus and minus readings in the field 
book, the reading of the ‘leading’ micrometer is assumed 
to be twenty revolutions at zero; so that all readings above 
twenty are plus, and all below minus. It will be observed 
that the reading is from the point marked on the stud to 
the terminal point on the tape—the measurement from 
a fixed point to one liable to fluctuation by temperature, being 
more reliable than were the measurement taken in the reverse 
”? 
way. 
