94 DR. FARADAY’S EXPERIMENTAL RESEARCHES IN ELECTRICITY. (SERIES XXVII.) 
places, even far north and south, this passage is thrown forward towards the astrono- 
mical meridian, either on the east side or the west, and comes into effect during the 
more influential hours of the sun or the cold. In all those places too where the dip 
is little, as at St. Helena, and in or near the sun’s path, it may be important in in- 
fluencing the amount of action. From the change of place of the sun between the 
tropics, and the variety of dip and declination at different places, the passing of the 
neutral planes by the sun and acting region must take place under an extreme variety 
of conditions ; the unravelling of which I think will be much assisted by knowledge 
such as that which the preceding experiments and principles give. The sun may 
be astronomically either north or south of the needle, and yet the declination of the 
needle not change in direction (2980.) ; or if there were much mean declination, as at 
Greenwich, then it might be astronomically east or west of it, and yet the declination 
produced not change its direction. The sun region may be south of a place and yet 
send its upper end further south (2990.) ; for all will depend upon its position in rela- 
tion to the magnetic meridian and the magnetic axis, which are in most cases very 
far removed from those that are astronomical : added to all these causes of varietv, 
there is the fixation of the lines of force in the earth (2919.), which tends to give a 
further diversity to them. 
3002. In the former paper I considered only the effect of air raised in temperature 
above the mean condition (2895.), illustrating it by the sun’s effect in the middle of 
the day ; now I purpose considering that which will be produced by the cold of night, 
which reduces the air of a given region below the mean air temperature of that place. 
When a portion of air is so cooled, its conduction power is increased ; in conjunction 
with the warmer air of surrounding regions it deflects the lines of magnetic force 
passing through both, as indicated by the type globe (2864. 2874.), and acquires 
what I have called conduction polarity (paramagnetic), meaning thereby simply that 
the lines of force draw together in the middle of the cooled air. 
3003. Theoretically, the effect of a cold region of air coming up from the east would 
be to make the magnetic lines of force, as they leave the earth, advance or bend to- 
wards it, because those in and about the cold air are inflected into it ; and as those 
immediately west of the cold region move into or towards it, so those further west, 
being in part relieved from their tension, will also move east, and thus an effect, the 
reverse of that of the sun (2877- 2972.), or the same as that of the helix in the para- 
magnetic position ( 2973 . 2994.), w’ill be produced. The upper ends of needles at 
places having dip show this deflection of the upper part of the lines of force, because 
they move by, with, and in them. 
3004. So as cold approaches, the lines will lean towards it until it is in the position 
of maximum action in the eastern quadrant; then they will return (in declination) 
before the cold, until both it and the line (or needle) are in the magnetic meridian ; 
after which, as the cold travels on westward, the needle will follow it west until the 
cold has attained its place of maximum action in the west quadrant (2982.) ; and then, 
