ATMOSPHERIC MAGNETISM — EXPERLMENTAL ILLUSTRATIONS. 
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the first gives places (for the helix) with a stable position for the needle, and the 
second such as have either stable or unstable positions, according to the helix 
distance. 
2992. If the helix be out of the plane and axis, then the end of the needle nearest 
to it leans from it as if repelled. If the helix be carried round in a circle of latitude, 
the end of the needle moves round before it just like the upper end of the needles at 
Hobarton and Toronto, in respect of the sun, during the midday hours. Instead of 
moving the helix round the needle, we may carry the needle into different positions 
as regards the helix, and then fig. 20 will represent the result. A pig. 20. 
result exceedingly simple, and in perfect accordance with the dia- 
magnetic disposition of the forces produced by the helix (29/2.), 
as the two dotted lines indicate. ® 
2993. As an expression of the facts for use in applying them to 
the explanation and illustration of natural- phenomena, it may be i 
said in respect of decUnation. that the lielix being above the needle 
in a plane having dip, and therefore above its magnetic e(|uator, if ^ 
on the east of a needle having north dip, it will send the south or 
upper end west, or if on the east of a needle having south dip (being 
of course then itself inverted (2972.)), it will cause the north or 
upper end to pass westward ; seeming to repel the end of the free needle or part of the 
line of force nearest to it. In reference to the mcUnation, it may be said, that the helix 
being above the needle, tends to send the upper end of the needle or line of force from 
it. If the helix is north of the magnetic axis, it will tend to send the upper end of the 
needle south ; if it is south, the upper end will go north. As in the case of the decli- 
nation, it is as if the end of the free needle or line of force nearest to it was repelled. 
In fact every case is included in this result, that if the helix be diamagnetically 
adjusted (2975.) for a free needle, whether it is above or below the needle, or on this 
side or that, the nearest end of the needle will be as if repelled, provided the helix is 
not in a neutral position. 
2994. I repeated all these experiments with the helix reversed, so as to give the 
effect of a paramagnetic globe of air (2865. 2973.). I need only say, that the effects 
were precisely the same in nature and order, only in the reverse direction. They will 
be required in the explication of the night and early morning actions, due to the 
cooling of the atmosphere (3003. 3010.). 
2995. In these experiments, that the laws of deflection might appear in their 
simplicity, the needle was suspended in the air, and the representation of the sun’s 
action carried round it in all directions. But in nature the aii- is only above the 
needle, and the earth as a magnet is beneath it. In the natural case also, there 
is the fixation of the lines in the earth (2919.), which tends, by holding them below 
the surface, to give them an amount of deflection at the surface, far beyond what 
they would have if they were as free to move in the earth beneath as in the space 
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