STANDARDS OF LINEAR MEASURE. 
367 
appeared not to differ sensibly from the Imperial standard yard, is in excess 
.0001945, or nearly two ten-thousandths of an inch. Now if the surface 
of the marble slab be supposed to be plane, the curvature of the Imperial 
standard yard would have the same effect in producing error as in the case 
of wires being placed under its ends ; and it will be seen that a curvature of 
the bar, the versed sine of which is only .002 of an inch in length, would be 
sufficient to produce the difference in question. 
On examining the preceding Table, it will be perceived that the difference 
between the length of the yard when lying flat on the marble, and its length 
when curved upwards, is much less than the difference when the bar is sup- 
ported at its ends, and consequently curved downwards ; and hence that the 
neutral plane is not as I supposed in the middle of the bar, but nearer to the 
convex surface: but on this important part of the subject I shall have to state 
more hereafter. 
Sir George Shuckburgh’s Scale. 
The bar upon which this scale is traced is 67-7 inches long, 1.4 inch wide, 
and 0.42 inch thick. The scale comprises 60 inches, the distance from the 
end of the bar to zero is 3.7 inches, and from 60 to the other extremity of 
the bar 4 inches. 
The scale being placed upon the marble slab, its ends projected two inches 
beyond it. I could perceive light between the bar and the marble, and could 
pass a sheet of letter-paper under the zero end to the distance of eight inches 
from its extremity, that is to the fourth inch of the scale. The bar was in con- 
tact with the marble for two inches and a quarter at the other end ; and from 
this part letter-paper could be passed between the bar and the marble for 
the distance of eighteen inches. The remainder of the bar rested upon 
the marble. 
In the following experiments, the wires in elevating the extremities were 
placed under 0 and 60 of the scale, and under 30 when the scale was to be 
curved upwards. 
3 B 
MDCCCXXX. 
