the Trigonometrical Operation . 133 
Feet. In. Pts. 
And hence there remains for the ultimate or 
true length of the bafe of verification, in the 
temperature of 62° of Fahrenheit’s thermo- 
meter, being the heat to which that on Houn- 
flow Heath was reduced, - - 28532.11.036 
Which make 28532,92 feet. 
Art. VIII. Remarks on the comparative accuracy of the two 
bafes . 
With regard to the accuracy of the meafurement of this 
bafe, compared with that executed on Hounflow Heath in 1784, 
from the infinite pains and care bellowed in both operations, 
it is very difficult to fay, to which the preference fhould be 
given. The expanfion of glafs being fo much lefs than that of 
Heel, if manageable glafs rods of equal length with the chain 
could have been obtained ; then, as far as that fingle circum- 
ftance might have affe&ed the refult, a meafurement made with 
fuch glafs rods would undoubtedly have deferved the pre- 
ference to one with the Heel chain. But when it is confidered, 
that the expanfion of Heel was determined by the pyrometer 
with the fame care as that of glafs ; that the wear of the chain 
is fo very fmall, as we have Ihewn it to be, in fix weeks ufe ; 
that coffers were laid for it, and its length transferred by means 
of the brafs fcales to the tops of immovable polls ; that, in the 
prefent cafe, there was but one-fifth part of the error arifing 
from faulty co-incidences as with the twenty-feet glafs rods ; 
■on this view of the matter, the preference feems to be due to 
the meafurement by the Heel chain, fuppofing always the 
error in excefs, caufed by the deviation from the aUignement- 
horizontally or vertically, to have affected both equally. 
As 
