to mcafure an Arc of the Meridian. 2 "3 



formed a point of departure und-er the fignal of Melun. Tue 

 rules 2, 3 and 4 were placed in a feries after the firft, and 

 brought into a line with the fight. The fmall rules, or flips, 

 were pufhed in to fill up the intervals •, and the lengths of 

 thefe flips, as well as the expanfion of each rule, and the 

 obfervations of the level, read by Delambre, were written, as 

 he dictated, by two affiftants, who each held a paper divided 

 into columns for that purpofe. After this the firlt rule was 

 removed and placed after the fourth rule, and the fame ope- 

 ration was repeated, directing the line always by the fight- 

 points, removed fucceffively at the diftance of every hundred 

 toifes, and taking care at each change of the rule to read the 

 expanfion, the length of the flip, and the two obfervations of 

 the level. 



This operation being repeated 3021 times, rendered it 

 confequently neceffary to make 6042 obfervations by the mi- 

 crofcope, and as many of the level. This was continued 

 without interruption during the day 5 and when night.obliged 

 the operators to fufpend their labour, the following method 

 was employed to fix the point where they left off, and which 

 was to ferve next morning for the point of departure. To- 

 wards the extremity of the laft rule an iron flake was funk 

 into the ground, and a 'pretty deep hole dug around it. The 

 iron ftake was then removed, and its place fupplied by a 

 wooden ftake, on the furface of which was nailed a plate of 

 lead. The laft rule was placed in fuch a manner that a 

 plummet, nicely adjufted and fufpended from its extremity, 

 ihould fall on this plate -, the point where it touched the lead 

 was marked as exactly as poffible, and the wooden ftake 

 was defended by a ftrong covering of wood, which was itfelf 

 covered with earth, fo that carriages could pafs over it with- 

 out caufing any derangement. Things remained in this ftate 

 till next morning, when the operations were refumed in the 

 like manner a:; that of the firft day. The whole operation 

 lafted forty days, three of which only were interrupted. 

 The operators generally advanced eighty-eight lengths of 



Vol. I. T the 



