342 MEASUEEMENT OF AN ARC ON THE 



When these stones were firmly fixed small silken- 

 threads were drawn across each other in the diagonals 

 of the square. A plummet: (pointed) was then sus- 

 pended from the point of intersection of those 

 threads^ and tiiey were so moved that the point of the 

 plummet coincided with the center of the nail in 

 the picket. The position of these threads being de- 

 termined, marks were inserted in the stone. The 

 cavity was then filled up, and a square thick stone 

 was fixed in the middle of the mass, having a circu- 

 lar place of about four inches diameter, sunk half an 

 inch deep, and whose center was marked by a point. 

 This point, by moving the stone and again applying 

 the silken threads was brought to coincide with the 

 point of intersection^ and then it was firmly fixed and 

 pointed. 



Precisely the same kind of building was erected at 

 the beginning of the base, but in place of having a 

 picket in the center, four large hooped ones were 

 driven into the ground, forming a square of about 

 ten feetj the small bamboo picket being intended as 

 the center. Silken threads were then drawn across 

 from the diagonal pickets, and so moved, that the 

 plummet first used, suspended from the .point of in- 

 tersection of the threads, might drop into the cavity 

 of the bamboo. That being adjusted, lines were 

 drawn on the tops of the pickets where the threads 

 had been extended. The building was then erected, 

 and the center both of the second and last tier, was 

 marked by the intersection of those threads when ap- 

 plied to the marks on the pickets. 



Such has been the mode of defining the extremi- 

 ties of the line. The buildings are well built of 

 stone and some brick, and will remain for years, if 

 not injured by acts of violence. They are intended 

 to receive an instrument on the top, and the points 

 are points of reference if it should ever be thought 

 necessary to have recourse to them. 



