Trigonometrical Survey. 423 
their proper height. To determine this height more accu- 
rately, another spar, whose thickness 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 ap- 
peared in a right line. Whilst 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. 
3. The Chains . 
These chains were made by Mr. Ramsden, and are of simi- 
lar construction in the joints to that which he made for the 
Royal Society, described in the LXXVth Volume of the Phi- 
losophical Transactions ; but they differ from thaj in other re- 
spects. Instead of one hundred links, each of these new chains 
contains forty, of 2^ feet long. The link is in form of a pa- 
rallelopipedon, of half an inch square, which renders it consi- 
derably stronger than that of the Royal Society ; 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 upon 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 readily and 
accurately obtained. The two chains will hereafter be dis- 
tinguished 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 
3I 2 
