43 
new standards of weights and measures. 
lutely free from error. But a national standard should be 
accurately that which it professes to be. It is not enough to 
determine its error, as the record of this may in process of 
time be lost ; it therefore became necessary to devise a 
method by which any perceptible error in those standards 
which are the foundation of all the others, might ultimately 
be annihilated. 
The four standard yards which I am about to describe are 
of brass, one inch and a quarter wide, and half an inch thick. 
This thickness is the same as that of Sir G. Shuckburgh’s 
scale, and was chosen in order that both might be affected 
with equal readiness by any change of temperature ; for as 
the imperial standard yard of 1760 is one inch square, I 
thought it preferable to adjust the new standards by means 
of Sir G. Shuckburgh’s scale, which, as I have before re- 
marked, does not sensibly differ from it. 
A disk of gold being let into the surface near one extre- 
mity, a hole was drilled through the bar at the distance of 
thirty-six inches from the centre of the disk, and being made 
slightly conical, a plug of brass was ground in the hole so 
as to fit it perfectly. A gold disk was let into the top of the 
plug, and reduced to a level with the surface of the scale. 
The other end of the plug projected beneath the scale, and 
had a small hole through it to admit a wire, by means of 
which it might be turned round. A very fine deep dot was 
then made by Mr. Dollond upon each of the gold disks, as 
nearly as it could be done at the distance of thirty-six inches 
from each other, the dot upon the moveable disk not being 
exactly in its centre. 
