140 MEASUREMENT OF AN ARC ON THE 



try, a full account of which has been pubHshed in a 

 former volume of the Asiatic Researches. Some lit- 

 tle alterations have been made in the coffers ; that 

 is, they were all of the same length, and the whole 

 together about ninety- six feet, so as to give room 

 for the pickets with the brass register heads. Their 

 sides continued to the ends, and their depth on each 

 side was the same, for the purpose of being turned 

 every day that they might fall into a curve by their 

 oxvn weight and that of the chain. I also used tripods 

 with elevating screws in the center, for supporting 

 the coffers, making no other use of pickets than for 

 the drawing and weight posts, and for carrying the 

 registg: heads. The top of each stand on tripod was 

 a thick circular piece of wood fixed firmly to the 

 end of the elevating screw, and a slip of board was 

 fastened across the circular top, screwed into the 

 center, and allowed to turn round. When the ends 

 of two coffers were placed on the top piece, this slip 

 of hoard was admitted into the under part of each, 

 and prevented their sliding off, a precaution that was 

 very necessary on account of the high winds. 



The point of commencement of the base was had 

 by dropping a plummet, from the arrow of the chain 

 suspended by a silken thread. A long but small 

 bamboo picket had been driven into the ground till 

 its top was level with the surface, and the cavity of 

 the bamboo was such as just to receive the plummet, 

 and when the first chain was in the coffers, drawn 

 o:it by the weight at the opposite end, it was adjusted 

 by the finger screw at the drawing post in such a 

 manner that the plummet might hang suspended over 

 tiie cavity of the bamboo, while the thread was ap- 

 ])lied to the arrow. This was done within the ob- 

 serva'ory tent, that the plumb line might hang freely 

 \virIiour being disturbed by the wind. The ba,mboQ 

 picket was preserved with great care during the tima 

 1 was observing for the latitude, and was then pro- 



