3 
60 On the Measurement of a Base on Epping Plains. 
traverses and which determines the length of the oh ag eg 
i i i . W. Dean, 
divided. : 
The usual comparisons of the apparatus with the standard six 
metre bar, were made before and after the measurement to ascer- 
tain that no change had taken place in the length from damage 
by transportation, and to add to the results of former comparisons. 
[he measurement was begun at the west end of the line on ~ 
Saturday the 18th of July, but the next week proved so rainy 
that it was only resumed in earnest on Monday the 27th. ae 
The work of the first Saturday (24 tubes) was remeasured on’ — 
the following Monday with precisely the same result as to length, — 
the end of the second measurement falling on the marks which | 
had been hao as terminating the first, and which were fine dots _ 
upon the head of a copper nail, placed in a stake some eighteen 
inches in length driven into the ground until its head just pro- — 
jected above the surface. The position of the mark was deter 
mined and verified, as all others of the sort in our measurements, 
by using a transit placed at right angles to the line and at& 
moderate distance from it. is was on a descending slope OF 
the strongest grade adopted and there was a difference of tem- | 
perature of some five degrees in the two measurements. oe 
On Tuesday a length of eighteen tubes, which had been meas: 
ured on Monday was remeasured with an identical result. This | 
was on an ascending slope. On Monday the work was in part | 
reaching the east end of the base on Monday evening. Thu 
ype in the broken days, 5:4 miles were measured in eight 
ays. 
This time included the marking of five permanent points neat 
to the ends of the successive miles, where stone posts have since | 
been placed. The ends of the base will be marked by regular | 
monuments. The base of the monument at the west end iscut | 
from the ledge of rocks upon which the signal stands. sy 
