the Trigonometrical Operation. 
SECTION FIRST. 
Defcription of the apparatus made ufe of in the meafurement of 
the bafe of verification in Romney Marjh, with the hundred- 
feet feel chain, in the autumn of 1787, with the refill t of that 
operation. Reference to be had to Plates I. and II . ; and alfo 
to t he table cont anting the general detail of the meafurement. 
Article I. Preamble. 
IN the account of the meafurement of the bafe on Houn- 
flow Heath in 1784, which appeared in the Philofophicai 
Tranfafhons oi the fubfequent year, we had occafien to fhew, 
how very accurately diftances might be determined by the fteel 
chain, when applied in the ordinary way on the natural fur- 
face of the ground, if that furface happened to be tolerably 
fmooth, which was the cafe in the inftance alluded to. By the 
comparifon of the meafurement of a length of one thoufand 
feet with the glafs rods, and with the chain when ufed with 
an apparatus adapted to the purpofe, it further appeared, that 
the difference between the refults was fo very fmall as fcarcely 
to be difcernible, fince it would not have exceeded half an inch 
on the whole length of that bafe of 27404.7 feet. 
Having always confidered the experiment on Hounflow 
Heath, juft now mentioned, as pofitive proof of the excel- 
lency of the chain, it had been refolved on to apply it to the 
menfuration of the bafe of verification in Romney Marfh, even 
if no other reafons ■ had exifted to make that choice eligible. 
Vol. LXXX. R Rll , 
