56 Capt. Rater’s experiments for determining the 
could arise from a difference in the length of the pendulum 
-in its vertical and horizontal positions. 
The knife edges were fixed as nearly as could be done by 
mechanical means, at right angles to the bar of the pendu- 
lum ; but the bar being flexible, they would most probably, 
when the pendulum was extended for the purpose of mea- 
surement, be found to be not precisely parallel to each other, 
and would consequently require some adjustment. To effect 
this, two opposite screws were passed through the sides of 
the mahogany case, so as to act in a transverse direction 
against that extremity of the pendulum which was next the 
steelyard, and the microscopes being brought over the ex- 
treme points of the knife edges, alternately on either side of 
the bar, the requisite parallelism was readily obtained by means 
of the screws, sufficient room having been left in the maho- 
gany case for the very small motion of the extremity of the 
pendulum which might be found necessary. This arrange- 
ment is represented in Plate IV. Fig. 5. 
To obtain the distance between the knife edges, two diffe- 
rent methods were used. For the first, four rectangular 
pieces of brass were prepared, about half an inch square. 
Very near to the perfectly straight edge of each, a fine line 
was drawn, to be viewed through the microscope, and these 
lines were each crossed at right angles by two others, in- 
tended to indicate that part of the first line from which the 
measurements were to be taken. These pieces were marked 
A ,a, and B, h. 
The pieces A and a , being placed with their edges in con- 
tact, in which position they were kept by the pressure of a 
spring, the distance between the fine lines first drawn was 
