434 
The Account of a 
To compare this length of the base with that assigned by 
General Roy, it becomes necessary to rectify a small over- 
sight in the 4th step of the process published in the Philso- 
phical Transactions for 1785. 
The equation for 6° difference of temperature there speci- 
fied, should consist of the difference of the numbers for brass 
and glass, and not of that for brass alone, viz . ~ x 3 >38338 
— 1,41638 = 0,3864 feet instead of 1.6346, which makes the 
base 0,7082 feet too long. Therefore the length of the base, 
as measured by the glass rods, is 27404,0843 feet, being about 
2f inches less than by the above reduction ; consequently 
27404,2, the mean of the two results, may be taken as the true 
length of the base. 
art. vii. Mr. Ramsden's Method of ascertaining the actual 
Lengths of the Chains A and B. Tab. XLIV. 
These chains were originally compared with the brass points 
inserted in the stone coping of the wall of St. James's church- 
yard ; but the temperature at the time of that comparison was 
afterwards forgotten by Mr. Ramsden. After the mensura- 
tion on Hounslow Heath was finished, the chains were again 
compared with those points ; but the result did not prove to be 
satisfactory, as there were reasons for supposing that some al- 
teration had taken place in the length of the coping ; but, inde- 
pendent of this, the great irregularities between the joints of 
the stones, some of which projected half an inch above others, 
rendered it at best a very rude and inaccurate operation. Mr. 
Ramsden had points remaining on his great plank, which had 
been transferred from the brass standard, but as the plank 
