Meafurement of a Bafe on Houaflov^- Heath. 453 
ilatlon ; and in having recourfe to the general table of the bafe, 
for- altitude, temperature, or correction for expanlion, we are 
afcending from the bottom towards the top, contrarily to the 
order in which the 'operation with the deal rods was conducted. 
On the morning of the i 8th of Auguft, the Hands with the ' 
various parts of the apparatus being placed in the manner juft 
now defcribed, the operation was begun by bringing the fi rid 
end of the chain to coincide with the interfedlion on the tripod, 
anfwering to the. end of the 1370th deal rod, and 4.31 feet 
diftant from the center of the pipe terminating the north-weft’ 
extremity of the bafe. The chain being ftretched along its 
five coffers by the fourteen pounds weight fufpended over the ' 
pulley at the farther end, and the temperatures of the five ther- 
mometers being regiftered in a book kept for that purpofe, a' 
fine trace was made on a piece of card faftened under the fea- 
ther-edge at the farther handle, denoting the end of the fir ft 
hundred feet. The chain being then moved oil into the next”- 
five coffers, thofe that had been thus vacated were carried for- 
ward to prepare for the third chain length, and thereby permit- 
the firft fet of fhnds to be elevated for the reception of the* 
glafs rods ; and fo in fucceffion with the others. 
In this manner we proceeded, and in the courfe of the day' 
were only able to meafure the length of ten chains, or 1000 
feet, being the forty-fixth and forty-fifth hypqthenufes of the' 
bafe, the firft of 400 and the laft of 600 feet. Being arrived 
at this point if was found, ghat the fine line on the- brafs Aide,’ 
marking the extremity of the tenth chain, fell fhort of ano- 
ther fine line on the fame flide, denoting the end of the fiftieths 
glafs rod,’ juft two- tenths of an inch. Now it will appear 
hereafters, .when we come to fhew, by the experiments witht 
the pyrometer, .what the real contractions of the chain and- : 
