4 1 7 
Trigonometrical Survey. 
some proof of its accuracy, which consideration induced Ge- 
neral Roy to make a comparison between the glass rods and 
the steel chain, which Mr. Ramsden had made for the Royal 
Society. For this purpose a distance of one thousand feet was 
carefully measured with the rods and the chain. The result 
of these measurements appeared to be such as would have pro- 
duced a difference of little more than half an inch upon the 
whole base, had it been measured with each of them respec- 
tively. But notwithstanding the apparent degree of accuracy 
which this, or any other mode of measuring may be supposed 
capable of, yet it seems necessary that every base, intended to 
become the groundwork of such nice operations, ought always 
(when circumstances will permit) to be measured twice at 
least. 
The manner in which the glass rods were applied in the 
measurement, is supposed to have rendered the operation liable 
to some small errors, which lying different ways, might pos- 
sibly have counterbalanced each other, and produced a true 
result : but this supposition ought never to be admitted in 
experimental inquiries, unless such errors can be nearly esti- 
mated. The principal cause of error is supposed to arise from 
the ends of the two adjacent rods being made to rest on the 
same tressel ; because when the first rod is taken off, the face 
of the first tressel, being then pressed by the end of one rod 
only, will acquire a tendency to incline a little forward. The 
error arising from this cause will evidently tend to shorten the 
apparent base. 
Another source of error is supposed to arise from the casual 
deviation of the rods from a right line, in the direction of the 
base, tending to increase its apparent length. And a third 
