Trigonometrical Survey. 429 
obligingly complied with the request, and lent the tressels be- 
longing to the Royal Society ; a description of which may be 
seen in the LXXVth Vol. of the Philosophical Transactions. 
.As the upper part of the pipe at the north-west end of the 
base was found to be exceedingly rotten, it became necessary 
to saw off 13 inches of it, which left enough of the cylinder 
remaining to fix the brass cup in, as it had been originally 
bored to the depth of two feet. This cup, which was also 
lent by the Royal Society, being inserted in the pipe, fitted it 
exactly. 
On the 15th of August, having previously traced out the 
line of the base, by means of the transit instrument, the ope- 
ration commenced, in the presence of Sir Joseph Banks, Dr. 
Maskelyne, and several other members of the Royal Society. 
The following table, which contains the particulars of it 
will explain the order of time in which the different parts of 
the measurement were performed. As it would swell this 
table to a great extent, were the degrees shewn by the ther- 
mometers inserted therein, it has been considered as proper 
to give only their sum, which is sufficient for finding the cor- 
rection to be applied in the reduction of the base, on account 
of the lengthening or contracting of the chain by variation of 
temperature. It may, however, be remarked, that the five 
thermometers were laid close by the chain, and suffered to re- 
main till they had nearly the same temperature, when they 
were read off, and registered in a field-book, whilst an ob- 
server at each end of the chain preserved a perfect coincidence 
between the arrow and a particular division on the brass scale. 
When the sun shone out, the chain was covered with a white 
linen cloth, the ends of which were put over the openings of 
MDCCXCV, 3 K 
