BRUNTON COMPASS ATTACHMENT 1393 
using the instrument as a magnetometer, zero of the graduated circle is 
set exactly beneath the north index of the compass box. 
OPERATION OF INSTRUMENT 
The instrument may be used on a flat board or table or on an 18 X 
24-inch planetable, or it may be mounted on a special tripod. The writer 
has found the use of the 18 x 24-inch planetable the most satisfactory. 
As the planetable is usually part of the geologist’s equipment and if 
properly constructed is made of non-magnetic material, its use is recom- 
mended. 
The steps of setting up the instrument and determination of hori- 
zontal intensity are as follows. 
1. Place the ring on the base of the Brunton compass in the circle 
on the compass holder so that the projecting screw on the base in the 
northeast quadrant fits into the locating hole. Then clamp the compass 
firmly in the holder by means of the clamps and clamp screws engaging 
the slots on either side of the compass box. 
2. Attach the auxiliary arm to the compass holder and clamp it 
by means of the clamping screw, move the magnet carrier with the rack 
and pinion into the approximate working position, and clamp the rack 
to the arm with the lower screw. 
3. Level and approximately orient the instrument on the plane- 
table or tripod with the aid of the level bubbles in the compass. 
4. With the zero line of the graduated circle opposite the index, 
turn the whole instrument in a horizontal plane until the north end of 
the needle points exactly to the north point on the graduated circle. A 
small reading glass (containing no magnetic material) is of assistance. 
The auxiliary magnet should be at a distance of about 20 feet so that it 
can have no possible effect on the needle. 
5. After orienting as in step 4 with the needle at rest, place the 
auxiliary magnet in the magnet holder with the south-seeking end of 
the magnet toward the compass. This should be done carefully so as 
not to disturb the level or orientation of the instrument. 
6. Move the magnet carrier (with magnet in place) by means of 
the rack-and-pinion device until the north end of the needle is exactiy 
at the 30° mark. In this position the needle is perpendicular to the axis 
of the auxiliary magnet and the magnet is in position known as the first 
position of gauss. It is suggested that the operator hold one hand on 
the auxiliary arm between the compass and the magnet while moving 
the magnet holder so that the orientation shall not be disturbed. 
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