To navigate safely with map and compass, the needle needs to be weighted differently for use in the southern hemisphere from how it is weighted for use in the northern hemisphere.
Here in the northern hemisphere the lines of magnetic flux are dipping down to the magnetic north pole. To ensure the needle remains level and the bearing does not bind, the south end is weighted. If this type of compas is used in the southern hemisphere, where the lines of magnetic flux rise up towards the equator and the north, the needle bearing may bind.
A southern hemsphere compass is weighted at the north end of the needle.