THE NEW NATIONAL STANHAED OE LENGTH, AND ITS PEINCIPAL COPIES. 655 
G, G, the two micrometer-microscopes. They pass through channels cut in the sides 
of the transoms ; each is. carried by two horizontal Y’s which are cemented into the 
back of the channel, and is pressed into the Y’s by bars of brass whose ends are fixed 
by screws to the pomts of the Y’s. 
H, H are two lamps for throwing light upon the reflectors of the microscopes, and 
thus iliummating the defining-points of the standards under observation. 
II, II, two perforations in the slab DD, through which the light of the lamps passes 
to the microscope-reflectors. 
J, J, narrow plates of brass, attached I believe for no other purpose than to support 
the plates, which cover the open side of the channels in which the microscopes are fixed, 
and which protect the microscope-tubes from violence and from radiation. 
KK, KK, broad plates of brass below the transoms, which I believe were intended 
to intercept radiation between the transoms and the standards. 
L, L, cords which supported the plates K. 
M, the great travelling platform, on which the box carrying the standards was placed. 
The box is not shown in this diagram. 
N, N, the two side-wheels of the travelling platform. 
O, their common axletree. 
P, the tail of the travelling platform, with a wheel below its end, which is not seen 
in this diagram. This wheel, as well as N, N, roll on the masonry B. 
Q, Q, Q, Q,fom’ screws for vertical adjustment of the box which is placed on the platform. 
K, E, R, R, fom' screws for lateral adjustment of the box. 
S, S, two screws for longitudinal adjustment of the box. They were sometimes turned 
by hook’s-joint handles. 
T, T, &c., eyes for the fixing of cords, for cmveniently raising the platform. 
Figm'e 2 is a front \iew of the apparatus . 
D' is a small block of stone cemented to D and to E, apparently to connect them 
more flnnly. 
E' is a larger block cemented to D and to both the transoms E, apparently for the 
same purpose. 
The Y’s which caiTy the microscopes G are shown (concealed by stone) in this view. 
U is the third wheel of the travelling platform, carried by its tail P. 
V is the large cast-iron box or water-trough carried by the platform ; which was 
nearly filled with water, and into which was dropped the box containing the Standards 
to be compared. 
W, eyes for the fixing of cords. 
X, a tap for the discharge of the water. 
Figure 3, Plate XXIX., is a side view of the apparatus. 
J' is the plate which covers the microscope-tube. 
The bars which press the microscope-tube into the Y’s are shown in this view, con-, 
cealed only by the plate J'. 
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