ON THE COMPARISON OP LINE AND END STANDARDS. 247 
The abutting surfaces are flat discs 6. mm. in diameter, 
and the material is turned out of the central portions, so 
that only a ring about 0.7 mm. wide was in contact with the 
ends of the Committee metre. 
Fig. 8 shows the abutting pieces when in contact with one 
another, and Fig. 9 when they are in contact with the Com¬ 
mittee metre. 
In both cases the pieces were pressed in contact by means 
of weak springs. No attempt was made to measure this 
pressure, for the reason that some experiments which were 
made proved conclusively that the pressure could be varied 
considerably without appreciably affecting the distance be¬ 
tween the lines. 
Pig. 8. Fig. 9. 
In order to determine the distance between the lines on 
the abutting pieces when they are in contact with one an¬ 
other it is essential that the line surfaces be in the same 
plane. This was effected by the following simple device: 
Two small pieces of plate-glass G G (Fig. 10), about 1 by 1.5 
cm., were laid upon a large plate of glass with just sufficient 
space between the edges of the smaller pieces to admit the 
projecting rings of the abutting pieces when in contact. 
Plaster of Paris was then poured over the small glass pieces 
and allowed to harden. The pieces of glass were thus “ set ” 
in the plaster, and the whole was then separated from the 
large glass plate. The plaster of Paris was then trimmed 
with a knife, and it was likewise removed from between the 
imbedded pieces of glass, after which the device was exam¬ 
ined. Since the two imbedded pieces of glass were in con¬ 
tact with the flat glass plate referred to, they were necessarily 
in the same plane, unless disturbed by the liquid plaster of 
