ATMOSPHERIC MAGNETISM — EXPERIMENTAL ILLUSTRATIONS. 
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circuit. Thus, beginning with the helix in the neutral position over the needle and 
going round by west and below, and then upwards on the east side to its first position, 
the north end of the needle will first pass westward, then eastw^ard, then westward, 
after that eastward, and finally westward to its original or neutral position. 
2982. As the helix is carried from the neutral planes (2978.) into any of the qua- 
drants, the power of affecting the declination of the needle is first developed, and 
then increases every way, from the edges of the quadrant until it attains its maximum 
force at the middle. Hence the maximum deflection east or west is when the helix 
is in the middle of each quadrant. Therefore, when the helix is carried from the 
middle of one quadrant to the middle of the next, only one motion in the needle 
appears ; as for instance, an increasing westerly declination, though the direction of 
the declination in relation to the mean position has been reversed in that time, and 
there was a moment when the needle had no extra declination, but was in that mean 
position. So also as the helix moves over one quadrant from one neutral plane to 
another, though the declination of the needle produced by it has not changed in 
direction, but has been, for instance, all the time west, still the needle will have ex- 
hibited two motions, going first west during the increase of the power, and then east 
whilst it is diminishing ; and hence it is that though there are four departures of the 
needle from and return to the neutral or mean position, whilst the helix circum- 
scribes it in an east and west vertical plane (2981.), there are only two complete 
journeys of the needle. 
2983. The amount of the deflection diminishes as the distance of the helix from 
the needle increases ; and the contrary. 
2984. Two other needles were slung (2975.) very oblique to the magnetic axis, 
one with its north end upwards and the other with its north end downwards, and 
these were submitted to the action of the helix as the former had been (2978.). They 
were affected exactly in the same manner, showing no difference ; i. e. a given end 
always moved the same way for the same change in position of the helix. If the 
helix was very near, then one pole was a little more influenced than the other in 
certain positions; but its removal further off took away that difference (which is 
easily accounted for (2970.)) and produced pure results. The place of the helix 
above or below the prolongation of the line of the needle made no difference, pro- 
vided it was in the same place as regarded the magnetic equator of the earth’s lines 
of force passing through the needle. 
2985. For the purpose of establishing the nature of the action which such a helix, 
always in the given or diamagnetic position (2975.), would exert upon the inclination, 
a small dipping-needle was submitted to its action and the following results obtairied. 
The needle could move in the plane passing through the magnetic meridian of 
London. 
2986. There M^as no deflection of the needle when the helix was in the plane of the 
magnetic equator, or in a plane perpendicular to that containing the mechanical axis 
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