PROCEEDINGS OF SECTION E. 285 
Peters in Germany. In comparing the brass bar, having a 
square section of 13.3 mm. side, the following mode was 
adopted : — 
Invariably the comparison between brass bar and _pen- 
dulum has been carried out in such a way that, prior to the be- 
oinning of the series of observations, a comparison of the length 
of the pendulum with the brass bar was made, stating also the 
temperature of the pendulum and the brass bar. Next the 
brass bar was brought under the microscopes of the comparator 
end adjusted; after that the pendulum was placed in the very 
same position under the microscopes in which the brass bar for- 
merly had been. The microscopic differences were then read 
and noted. As soon as the pendulum was brought back again 
to the proper position for oscillation, the observations of 
the coincidences began to be made. After the series was 
finished the brass bar next again was brought under the micro- 
scopes of the comparator and adjusted ; after that the brass bar 
was removed and the pendulum brought in its place, whereupon 
the distance between the knife-edges was measured with the 
ereatest care. After that had been done and the pendulum re- 
moved, commenced the verification of the brass bar, which was 
for that purpose again placed under the micrometers. Every 
measurement of the distance between the knife-edges, measured 
at four different points along each knife-edge, was frequently 
repeated and verified at the commencement and the end of the 
comparison. In this way each measurement of the pendulum 
was included between two measurements of the brass bar. It 
is hardly necessary to mention that the temperature at various 
parts of the apparatus was carefully noted, in order that the 
necessary reduction to a normal temperature cay be after- 
wards ettected. 
The brass bar carried about 7 cm. from its extreme end two 
small brackets, by means of which the bar could be suspended 
in quite the same layers as the pendulum when ready for com- 
parison. The brass bar was brought into a vertical position 
by means of a small screw, which pressed gently against it near 
its lower extremity. The bar was quite freely suspended, and 
not supperted in any other way. The in such a way secured posi- 
tion of the brass bar was afterwards taken up in the same man- 
ner by the pendulum. In this way the exactity of the measure- 
ment was guaranteed as soon as the micrometers were properly 
adjusted. It required but little practice to execute the de- 
scribed operation with all degrees of exactness. 
In the case in which the comparator was mounted when not 
used, there was sideways a kind of support on which the brass 
bar could be hung up in a way ensuring its being continually in 
a like position, whether in use or put aside. 
