of an Arc of the Meridian. 419 
50-feet chain, were both compared with the standard B. For 
this purpose, a calm cloudy day was waited for, which oppor- 
tunity presented itself on the 2d of June. The pickets for the 
registered heads were then driven into the ground a considerable 
depth, and the coffers laid in a right line between them. The 
chain A was then laid out perfectly straight ; and five thermo- 
meters, equally distant from each other, were put close to its 
side, their temperatures being as follows. 
Thermometers. 
1 — 2 — 5 — 4 — 5. 
67° 65° 67°, 5 — 67°, 5 — 6 f, 5. 
The chain A was then taken out of the coffers, and B laid 
out in its stead. The difference of their lengths, which was mea- 
sured with the micrometer-screw, was found to be 1 revolution 
6 divisions, viz. A longer than B ; the temperature remaining 
constant the whole time of trial. In the course of the day, the 
same operation was repeated, the five thermometers standing at 
6 g°, 5 — 6 f — 69°, 5 — 6 g° — 6q°,when B was found to be 1 revolution 
6 \ divisions of the micrometer-head shorter than A. Therefore, 
the mean, viz. 1 revolution 6 + divisions, was considered as the 
true difference of their lengths. The length of twice the 50-feet 
chain was, at this trial, found to exceed that of B, 2 revolutions 
4,5 divisions ; which is nearly the same determination as formerly 
resulted, from a comparison of the chains with each other on 
Sedgemoor. It may be seen too, by referring to the account of 
the measurement of the base on that spot, that the difference 
between the lengths of the standard B and common chain 
A, was nearly the same at that period as now ; the difference 
being 1 revolution 7 divisions. I therefore concluded I might. 
