Mr Christie's Observations 
thesis, that the needle was guided in its horizontal direction, by 
magnetic particles in a line passing through its centre, in the di- 
rection of the dip, and that the iron acted principally, if not 
wholly, on these particles, causing, by their deviation towards 
it, a corresponding deviation of the horizontal needle. 
In the different experiments which I made, the apparatus 
consisted of a cast-iron ball 12.78 inches in diameter, suspended 
over the centre of a table, in the construction of which iron was 
carefully excluded. In the middle of the table a circular hole 
was cut, 13.25 inches in diameter, so that the ball could be let 
down below the plane of the table, or raised above it, by means 
of a system of pullies. After the table was rendered perfectly 
steady and horizontal, the magnetic meridian was accurately as- 
certained, and being drawn, the table was divided at every 10°, 
reckoning from the meridian, by lines drawn from the centre to 
the circumference. The compass being placed on one of the 
divisions, so that its north and south line coincided exactly with 
that division, and its centre was at the distance of 12 inches 
from the centre of the table, the ball was raised until it appear- 
ed to have no influence on the needle : it was then lowered, 
inch by inch, and the deviations at every inch carefully noted, 
until the ball had descended so far below the table as to cease 
influencing the needle. This was done with two compasses, at 
every 10° from the north to the west, and from the south to 
the east. NESW, represents the plane of the table, the centre 
of which is O. NOS is the magnetic meridian, and EOW at 
right angles to it, passing through the east and west points ; C 
/p is the centre of the magnetic 
needfe, and sCn, in a vertical 
^ / plane, parallel to SN, the di- 
rection of the dipping needle, 
in which I suppose magnetic 
particles to act upon the poles 
of the horizontal needle. CL, 
parallel to NS, is the line in 
which the needle points when 
influenced by the ball. OZ 
is a vertical line, from the centre of the table, in which the' 
