THE NEW NATIONAL STANDARD OF LENGTH, AND ITS PEINCIPAL COPIES. 657 
Figure 6, Plate XXX., is merely an enlarged view of the adjustment of the stirrup- 
support in figure 5. 
Figure 7 is a transversal section of the water-trough, drop-box, quicksilver-box, and 
bars. 
k is the longitudinal partition of the quicksilver-box. 
Figure 8 is a plan of two standards as placed in the quicksilver- trough for comparison. 
In figure 4 they are concealed by the cover Z. 
Figure 9 is an enlarged plan of the adjustment of the stirrup-supports in figure 8. 
Figure 10 is a longitudinal section of the mounting employed when the standard-bars 
are supported by “ block-and-stirrup.” 
1 is a block, planted on the platform M. 
m, a wooden box. 
n, n, two blocks. 
o, a brass plate resting upon them. 
p, a small metallic block (with foot-screws), on which the standard-bar immediately 
rests. This is the “block” of the “block-and-stirrup” apparatus. 
q, one of two uprights supported by a pedestal similar to that of p. 
r, a stirrup swinging with pivots on the top of q. This is the “ stirrup ” of the 
“ block-and-stirrup ” apparatus. The standard-bar rests in this stirrup. 
Figure 11 is a plan of the instrumental arrangement for the comparison of the length 
of one standard-bar defined by line-measure with the sum of the lengths of two sections 
of two standard-bars defined by end-measure. The principle concerned in this operation 
will be explained in the latter part of this Memoir. 
s and t are two end-bars, with then’ ends in contact. The line-bar h, and each of the 
end-bars s and t, is supported upon a lever-frame, to be described shortly. 
w, w, &c. are collets, fixed near the extremities of the end-bars by stiff-friction only. 
X, X are spiral springs attached to the collets, drawing the ends of the bars into con- 
tact, "with a pressure of about 3 lbs. 
Figure 12 is a longitudinal section of the same arrangement, showing however only 
the two end-bars upon their lever-frames. 
V, V, V, V are insertions of pieces of agate in the ends of the metallic bars. 
For understanding of the supporting-mechanism, it must be remarked that each bar 
rests upon two lever-frames ; and that the form of each lever-frame is the following. 
The foundation-block supports the centre of motion at the centre of the length of a lever 
of first order ; this lever consists of two parallel plates separated by a much larger space 
than the breadth of the bar, and the parallel plates are tied by cross-ties. This lever 
carries the two centres of motions of two similar but smaller levers of second order, 
each consisting of two parallel plates, falling between the parallel plates of the lever of 
