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to carry the needle with safety, will be found to give a 
quantity of torsion so small that it may be neglected, not 
amounting to more than a small fraction of a minute in a 
deflection of 45°. For suspending the deflecting bars a 
stronger thread is required, but by increasing its length to 
four or or five feet the effect of torsion in this latter case 
also becomes too small to require notice. 
The remaining part of the apparatus gives the means of 
placing the deflecting bars at accurately measured distances 
from the needle. This has generally been done by means 
of a divided metal rod; but I find that a plain board of 
well-seasoned deal, with a circular hollow excavation in the 
centre for the graduated circle, answers the purpose very 
well. It is needful to be able to place the bars rapidly to 
an exact position on the board. This is best done for each 
distance by driving pins into the board, and cutting them off 
pretty close to the wood, one pin regulating the distance 
and two others the direction. It is only necessary to push 
the bar up to its defined position. The distances of the 
pins on the east side of the suspended needle from The 
corresponding ones on the west side are in my own apparatus 
15, 20, 25, and 30 inches/half those distances being the 
mean distances of the bar from the needle. The first two 
distances are, however, all that are actually required. 
In making the observations of deflection two bars are 
used, one on the east side, the other on the west of the 
needle. They must be carried to their respective places by 
wooden pincers, and on no account touched by the fingers 
or subjected to any violence. The deflection marked by the 
pointer at both ends of the needle must be observed, and 
then the bars must be reversed and the deflection in the 
other direction also observed. The bars must then be 
removed to the next distances and the observations repeated. 
Finally the bars must be made to change sides from east to 
west, and all the former observations repeated in the reverse 
