4i 8 Tfje Account of a 
error is supposed to result from the method which was used, 
of supporting the ends of the rods on two tressels only, by 
which they become liable to bend in the middle. This con- 
cave form of, the rods would also tend to lengthen the base. 
The first of these causes of error was submitted to experi- 
mental inquiry in the garden of Richmond house, Whitehall, 
in the presence of his Grace the Duke of Richmond, Sir Jo- 
seph Banks, Mr. Ramsden, and Mr. Dalby; when it ap- 
peared evidently, that the glass rod had a small motion when 
the other rod, which had counterbalanced it, was taken from 
the tressels. 
These considerations, therefore, rendered it necessary to 
compare the measurement with the glass rods, with that 
performed by some other method ; not on account of any 
doubt being entertained of the care with which General 
Roy's operation had been performed, but solely with a view 
to bring this new mode of measuring to some proper test. No 
method of comparison could, perhaps, be better than measur- 
ing the same base with the steel chain. General Roy himself, 
in his remarks on the comparative accuracy of the two bases, 
that of Hounslow Heath and Romney Marsh, evidently gives 
the preference to the chain ; which, every circumstance con- 
sidered, it is certainly right to do. These reasons induced his 
Grace the Duke of Richmond to direct the base on Hounslow 
Heath to be remeasured with the steel chain ; and although 
the result does not differ from the glass rods by so small a 
quantity as General Roy's experiment assigned, yet it does 
not amount to more than three inches on a base exceeding 
five miles. 
