Mr. Christie on the diurnal deviations , &c. 343 
two, and still more so if these were placed in the line of the 
dip. According to the manner in which I have for a long 
time viewed the nature of the forces which give direction to 
the horizontal needle, and their disturbance by other forces, 
it appeared to me that, by applying two magnets to the needle 
in the line which it would take if freely suspended by its centre 
of gravity, but having their poles in the reverse position to 
those of the needle, one above and the other below its centre, 
a portion of the forces acting upon the horizontal needle in 
the line of these magnets, or of the dip, would be destroyed ; 
and it would therefore still be acted upon by forces in the 
same direction as before, but of less intensity : whereas by 
even applying the poles of two magnets to the corresponding 
poles of the needle, and in the same plane with them, the 
horizontal directive force of the needle would be diminished, 
by increasing the angle which the resultant of the terrestrial 
forces, and those of the magnet made with the horizon ; and 
which would be nearly equivalent to increasing the angle of 
the dip. This, however, will not be quite correct when the 
length of the needle bears a sensible ratio to the distances of 
the magnets ; but, when this adjustment occurred to me, I 
was not aware that the magnets must be brought so near to 
the needle as I afterwards found necessary. Notwithstanding 
this, as I considered the arrangement to be a good one when 
the observations were to be made with the needle in the 
meridian, I adjusted, in the line of the dip, two powerful bar 
magnets, each twelve inches long, .95 inch wide, and .375 
inch thick, to an instrument with which I had made a variety 
of magnetical experiments ; one above the needle with its 
north pole towards the centre, to destroy a portion of the 
