246 
FISCHER. 
tance between the 0 and 36th-inch stop, and it only remains 
to determine this distance, which may easily be done by com¬ 
paring it with a standard yard. 
The auxiliary pieces just described suggested a method for 
comparing the Committee metre, also depending upon the use 
of abutting pieces, the construction of which, however, re¬ 
quired considerable mechanical skill. The principal require¬ 
ment was that the lines ruled upon them should be so close 
to the abutting surfaces that when the pieces were in contact 
with one another the lines on both should be visible in the 
field of one of the Comparator microscopes. The distance be¬ 
tween the lines could thus be measured with the micrometer 
screws. This condition would also make it possible to directly 
compare the combined length of the Committee metre and 
abutting pieces with Prototype No. 21. 
Fig. 6. 
Fig. 7. 
Two platinum pieces meeting all requirements were con¬ 
structed in the Instrument Shop of the Coast and Geodetic 
Survey, of which Fig. 6 is a perspective and Fig. 7 a side 
view. 
Each of these pieces carries upon its horizontal surface a 
single line ruled parallel to the abutting surface and only 
0.8 of a millimetre from it. In addition, two parallel lines 
about 0.5 mm. apart are ruled perpendicular to the contact 
surface and across the first line. All observations were made 
on that portion of the single line between the two parallel 
ones. 
