416 The Account of a 
concurring to promote the further execution of a design of 
such great utility, as well as honour, to the nation, his Grace, 
with his Majesty's approbation, immediately gave directions 
to prepare all the necessary apparatus for the purpose, which 
was accordingly provided in the most ample manner. 
SECTION FIRST. 
An Account of the Measurement of a Base on Hounslow Heath, 
with an hundred Feet Steel Chain , in the Summer of the Tear 
1791, Reference to be had to Tab. XLIII. and XLIV. 
article i. Preamble. 
Previous to entering upon the ensuing account, it may not, 
perhaps, be improper to enumerate some preliminary matters 
relative to the subject. The first mode of mensuration adopted 
by General Roy was that with deal rods, which had also been 
used and approved of in other countries. In the course of the 
measurement, however, it appeared, that the sudden and irre- 
gular changes which these rods were liable to, from dryness, 
humidity, or other causes, rendered them totally unfit for as- 
certaining the length of the base with that degree of precision, 
of which it was at first thought they were capable. On this 
account they were laid aside, and glass rods substituted in their 
stead. These rods were contrived with great ingenuity to 
answer the purpose, as fully appears by the account given of 
them in the Philosophical Transactions. But this mode of 
mensuration being the first of the kind, seemed to require 
