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with a brass washer and locking-nut. By this means the box can be moved 
through a small distance in the north and south direction, and securely 
clamped in position. On the upper surface of the box is fastened a plate of 
glass on which stands the magnetometer proper. This part of the instru- 
ment is also constructed of mahogany. A wooden pillar 20 cm. in height 
has a narrow hole drilled longitudinally down through it. This hole 
terminates in a small cell with a glass window in front. The cell is just 
large enough to contain the mirror of the magnetometer — a concave mirror, 
1 cm. in diameter, having a focal length of 50 cm. The mirror has attached 
to its back a small piece of magnetised watch-spring about 8 mm. in length. 
The needle and mirror are suspended by a tine quartz fibre from a screw at 
Fig. 3. — Plan of the Magnetometer. 
the top of the upright pillar of the magnetometer. By means of this screw, 
the axis of which is vertical, all torsion can be removed from the fibre when 
the needle is hanging in its equilibrium position ; and by giving the screw 
an observed number of complete turns a determination of the torsional 
rigidity of the fibre can be made. The pillar of the magnetometer is 
attached to a circular base provided with three small brass levelling-screws. 
The position of these feet on the glass top of the box-stand is defined by 
the hole, slot, and plane method. 
A A (fig. 6) is the magnetising solenoid. Two brass tubes 45 cm. in 
length are connected at their ends by brass rings so as to form a water- 
jacket B B measuring 4 cm. in internal and 6 cm. in external diameter. 
On the outside of this is wound 868 turns of No. 15 s.w.g. copper wire in 
four layers (only one layer is shown in the figure). The wire is double 
silk-covered, and each layer is varnished over after winding. The terminals 
