480 The Account of a 
with one of the divisions near the end of the brass scale, by 
which any error arising from their not being exactly in the 
same vertical plane, was rendered so trifling as not to be worth 
notice. 
Having now related, with as much conciseness as the sub- 
ject will admit, the methods which were adopted for the exe- 
cution of the most essential parts of this operation, there re- 
main only a few other particulars to be related before we give 
the reduction of the base. 
After as many points as were judged necessary had been 
fixed in the true direction, by the means heretofore described, 
and the chains compared with each other, the mensuration was 
begun, and continued without much interruption for seven 
weeks, when it was finished with that part of the 366th chain 
which terminated its apparent length. 
The method taken to mark this last mentioned chain, was 
by cutting a small hole in ihe bottom of the coffer, through 
which a plumb-line was made to pass, the point of the plum- 
met being brought over the end of the. base, and the chain 
moved till it touched the wire; a slight scratch was then made 
with a file at the point of contact. 
On the first favourable opportunity, subsequent to this con- 
clusion of the measurement, the chains A and B were com- 
pared with each other, when it was found that the wear of the 
former, by the constant use of it, was only one division of the 
micrometer head, or ^|^th of an inch. The smallness of this 
quantity in the measurement of a base of such great length, 
was doubtless owing to the pivots, aijd pivot holes of the joints 
being smoothed, and as it were polished, in the operation on 
Hounslow Heath ; and it may also be adduced as some proof. 
