435 
Trigonometrical Survey. 
itself was found to be subject to a daily expansion and con- 
traction, he turned his thoughts to the invention of some other 
method of measuring the lengths of the chains, in a more un- 
exceptionable manner. 
On considering that the expansion of cast iron is nearly the 
same as that of the steel chain, he procured a prismatic bar of 
that metal, of 21 feet long, judging it to be the most proper 
material for the present occasion, as well as for establishing a 
permanent standard for future comparisons of the same kind. 
The manner in which the bar was fitted up for the purpose 
will be readily understood by attending to Tab. XLIV. 
The great plank was cut to the length of about 22 feet, and 
on one of its narrow edges 21 brackets were fixed ; each of 
which had a triangular notch to receive and support the bar, 
with one of its angles downwards, so that the upper surface 
became one of the faces of the prism. Before the brass points 
were inserted in this bar, Mr. Ramsden compared his brass 
standard with that belonging to the Royal Society, for which 
purpose, on Nov. 22d, 1791, it was sent to their apartments in 
Somerset house, where, after the two standards had remained 
together about 24 hours, they were found to be precisely of the 
same length. Brass points were then inserted in the upper 
surface of the bar, from Mr. Ramsden's standard, at the dis- 
tance of forty inches from each other, the whole length of 20 
feet being laid off on those points in the temperature of 54 0 . 
The chains were measured in the Duke of Marlborough's 
riding-house, where the light was very convenient for the pur- 
pose, and the whole apparatus was sheltered from the wind and 
sun. The plank and bar were supported on five of the tressels, 
or tripods, belonging to the Royal Society, and the upper sur- 
