45 
new standards of weights and measures. 
not until after several days that I discovered the cause of 
this perplexing circumstance. I found that by placing a card, 
the thickness of which was accurately one-fiftieth of an inch, 
under the middle of the standard, the distance of the dots 
was much increased, and by placing a card of the same 
thickness under each of the extremities, and withdrawing 
that which was under the centre, the distance of the dots 
was considerably diminished. The total difference amounted 
to no less than ,0016 of an inch, whilst the double of the 
error which would have arisen from mere curvature under 
similar circumstances would not have been one ten-thou- 
sandth of an inch. 
The cause was now evident ; by elevating the middle of 
the standard, the under surface was shortened, and the upper 
surface extended ; and on the contrary, when the extremities 
were elevated the upper surface was compressed and the 
lower surface lengthened ; the quantity of the effect evidently 
depending upon the thickness of the bar. 
Having thus assured myself of the source of the error, 
a method of obviating it soon presented itself. As the upper 
and under surfaces of the bar are in different states, the one 
being compressed and the other extended, there must be an 
intermediate plane which suffers neither extension nor com- 
pression, and this plane must be nearly midway between the 
two surfaces. I therefore caused Mr. Dollond to reduce 
the thickness of the bar for the distance of an inch and three 
quarters from its extremities to one half ; the gold disks and 
plugs were then inserted as before, and the adjustment 
completed in the manner which has been described. The 
plugs being secured, and the projecting parts removed, the 
