55 
length of the pendulum vibrating seconds. 
The standard yard made by Bird in 1758, for the House 
of Commons, better known by the name of Bird’s Parlia- 
mentary standard, is little adapted for measurements where 
great precision is necessary. The yard is determined by 
two large dots made on gold pins, which are let into a bar 
of brass. The mean of a number of bisections of these dots 
gave their distance equal to 36,00016 inches of Sir George 
Shuckrurgh’s scale. 
Measurement of the pendulum. 
The pendulum was let into a solid piece of mahogany 
edgewise, to such a depth that the knife edges were about 
one twentieth of an inch above its surface. To one end of 
the pendulum, a common spring steelyard was attached by 
its hook, and a string being passed through the ring, and 
fastened to an upright piece of wood screwed to the end of 
the mahogany case, the pendulum was extended by a force 
rather greater than its own weight (about ten pounds), and 
consequently, no error (if any such were to be apprehended) 
ceeded that of the last by ,00017 of an inch. Imagining that the difference between 
Sir George Shuckburgh’s result and mine, might possibly be occasioned by an 
error in the divisions bounding that part of General Roy’s scale which I had em- 
ployed, I compared it with various other portions, and found no greater difference 
than might have been expected from unavoidable imperfection of division. It is to 
be presumed then, that the error into which Sir George Shuckburgh appears to 
have fallen, must have arisen from the two scales not having been of the same tempe- 
rature at the time they were compared, particularly as Sir George Shuckburgh’s 
is by far the most massive of the two. I may here add, that last winter wishing to 
know whether the expansion of the two scales was equal, I roughly compared them 
together once, at the temperature of 33 0 , when it appeared that 42 inches on General 
Roy’s scale, was equal to about 42,001 inches of Sir George Shuckburgh’s 
standard. 
