Trigonometrical Survey. 425 
each of the divisions into 4 parts, the micrometer will measure 
the Yo W -h an tolerably exact. 
For finding the relative Lengths of the Chains. 
In order to accomplish these experiments in the most un- 
exceptionable manner, after the chain was properly stretched 
in the coffers, and the thermometers placed by it, the whole 
remained till all the thermometers stood steadily at the same 
height. The ends of the chain being then in perfect coinci- 
dence with particular divisions on the brass register-heads, the 
chain was quickly taken out and replaced by the other, which 
being properly stretched in a right line, and a coincidence 
made at the drawing-post end of the chain, the variation of 
the other end from the division on its register- head showed 
the difference of the lengths of the chains, which was measured 
by the micrometer. As it required weather particularly steady 
to succeed in these experiments, we were obliged to catch the 
most favourable opportunities that presented themselves, which 
happened on the 29th and 30th of July ; on those days the 
chains were compared with each other, and the results were 
as follow. 
July 29th. Thermometers remaining steadily at 75 0 during 
and after the operation. 
The chain B was found to be divisions of the micrometer 
head shorter than the chain A ; and on being shifted, A was 
found to exceed B 6 j- divisions. 
Same day. Thermometers steady at 67-j 5 . 
The chain B 6 divisions shorter than A ; and being shifted, 
the chain A was 6 divisions longer than B. The mean from 
these experiments is, A 6 ± divisions longer than B. 
