[ 501 ] 
ridian, or turned half round for repeating the ohferva- 
tions with the plane of the inlfrument placed the contrary 
way, in order to find the error of the line of collimation. 
A large parallelopiped tent, 1 5^ feet fquare and 17 
feet high, was alfo provided for flieltering both the in- 
Itrument and the obferver who fliould ufe it, compofed 
of joices of wood well framed together, and covered with 
painted canvas. The Society was likewife poflefled of 
mofl: of the other inftruments requifite for this experi- 
ment; as an aftronomical quadrant and tranfit inftru- 
ment made by Mr. bird, and an aftronomical clock by 
SHELTON, which had all been provided on occafion of 
the obfervations of the tranfit of Venus iniybi or 1769. 
A theodolite of the heft fort was wanting, a neceftary in- 
ftrument for obtaining the figure and dimenfions of the 
hill. One of Mr. ramsden’s conftru(ftion of 9 inches dia- 
meter, was thought the fitteft for the purpofe, on account 
of the excellence of the plan on which it was made, and 
the number of its adjuftments, being capable of meafur- 
ing angles for the moft part to the exaclnefs of a fingle 
minute. The other inftruments prepai'ed for this buli- 
nefs were, two barometers of M. be lug’s conftrucftion, 
made by Mr. nairne; a common Gunter’s chain; a roll 
of painted tape three poles long, having feet and inches 
marked upon it; two fir poles of 20 feet each, and four 
wooden ftands, for fupporting them when ufed in mea- 
Turing the bafes, and a brafs ftandard of five feet for ad- 
jufting them. The poles and ftands were provided on 
the fpot. 
Although 
