Meafurement of a Bnfe on Hounflow-Heath. 45^ 



i^-^tion ; and in having recourfe to the general table of the bafe, 

 f©r altitude, temperature, or corredlion for expanfion, we are 

 afcendi tig from the bottom towards the top, contrarily to the 

 order in which the operation with the deal rods was conducfled. 



On the morning of the 18th of Auguft, the (lands with the 

 various parts of the a.pparatus being placed in the manner juft 

 now defcribed, the operation was begun by bringing the firfl; 

 end of the chain to coincide with the interfedion on the tripod, 

 anfvvering to the end of the i3^oth deal rod,, and 4.^1 feet 

 diftant from the center of the pipe terminating the north-weft 

 extremity of the bafe. The chain being ftretched along its 

 five cotters 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 firil:' 

 hundred feet. The chain being then moved on 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 firfl: fet of ftands to be elevated for the reception of the- 

 glafs rods ; and fo in fucceflion with the others. 



In this manner we proceeded, and in the courfe of the day 

 were only able to meafurt the length of ten chains, or loco 

 feet, being the forty-fixth and forty-fifth hypothenufes of the 

 bafe, the firft of 400 and the laft of 600 feet* Being arrived 

 at this point it was found, that the fine line on the brafs flide, 

 marking the extremity of the tenth chain, fell fliort of ano- 

 ther fine line on the fame flide, denoting the end of the fiftieth 

 glafs rod, juft two-tenths of an inch. Now it will appear 

 hereafter, when we come to fhew, by the experiments with 

 the pyrometer, what the real contradions of the chain and 



glafs 



