6l8 PHILOSOPHICAL TRANSACTIONS. [aNNO 1795. 



cope directed to it, while the intermediate pickets were driven to their proper 

 height. To determine this height more accurately, another spar, whose thick- 

 ness was equal to the height of the axis of the telescope above the top of the 

 picket, which supported it, was repeatedly laid on the top of each picket at the 

 time of driving it, till its upper edge and the fixed spar appeared in a right line. 

 While the pickets were driving, they were moved a little to the right or left, as 

 directed by signals from the observer at the telescope, till their tops appeared in 

 the same right line. 



The chains were made by Mr. Ramsden, and are of similar construction, in 

 the joints, to that which he made for the r. s., described in the 75th volume of 

 the Philos. Trans. ; but they differ from that in other respects. Instead of 100 

 links, each of these new chains contains 40, of 2-1- feet long. The link is in 

 form of a parallelopipedon, of half an inch square, which renders it considerably 

 stronger than that of the r. s.; and the chain, having fewer links, becomes less 

 liable to apply itself to any irregularities which the coffers may be subject to. 

 The handles are of brass, and being perfectly flat on the under side, they move 

 freely on the brass register-heads, by which means the coincidence between the 

 arrows at the extremities of the chain, and the divisions on the scales, are rea- 

 dily and accurately obtained. The 2 chains will hereafter be distinguished by 

 the letters a and b. 



On Saturday July the 23d, all the foregoing articles were conveyed from the 

 Tower to the end of the base near King's Arbour, where tents were pitched for a 

 party of the royal regiment of artillery, consisting of 1 serjeant and JO gunners, 

 who were to be employed in the laborious part of the operation. 



To ascertain the relative lengths of the chains, 2 strong oaken pickets were 

 driven 2 feet into the very firm ground, and the drawing-post was made fast to 

 them. Five coffers were arranged in a right line, and supported on courses of 

 bricks. The chain was then placed in the coffers, and stretched with a weight 

 of 55 lbs. Notwithstanding the great resistance which it was thought these 

 pickets were capable of, yet it was found insufficient to counteract the friction 

 between the coffers and the chain, when the expansion or contraction took 

 place. Three pickets therefore, of 44 inches long, were driven into the 

 ground, within 6 inches of their tops, and the drawing-post was fastened to 

 them by several folds of strong rope. The pickets and rope were also covered 

 with earth, to prevent their being warped by the sun. The micrometer-screw, 

 attached to the brass register-head, by means of which the expansion or con- 

 traction was measured, contained 26 threads in an inch. The circular head was 

 divided into 10 equal parts, and consequently each division will measure TTB-j-th, 

 part of an inch. But as the eye readily subdivides each of the divisions into 4 

 parts, the micrometer will measure the -roWth of an inch tolerably exact. 



