ATMOSPHERIC MAGNETISM — DIURNAL VARIATION — DIRECTION. 
65 
heated, will, because of its being a worse magnetic conductor than the latter (2861. 
2862.), tend to open out (2807.) ; and the mass of heated 
air, as a whole, will assume the condition of diamagnetic 
polarity. If, therefore, for the sake of simplicity, the mag- 
netic and astronomical poles of our earth be supposed as 
coincident, and fig. 14 represent a section taken through 
them and the place of the sun, then N and S will be the 
magnetic poles, and the different curves cutting the outline 
of the circle will sufficiently represent the course of the 
magnetic lines as they occur at or about the surface of the 
earth, H being the sun, and a the place immediately beneath 
it, which is also coincident with the magnetic equator. By 
this diagram we shall have an illustration of the hypothe- 
tical effect on the inclination of the needle. 
2921. Considering the point a first, and assuming as yet 
that the maximum of change in the air is always at the 
surface of the earth, we shall find that there the lines of force will open out, pre- 
serving in some degree their parallel or concentric relation. Consequently a mag- 
netic needle, free to move in every direction, and therefore taking up its position 
in the line of force, ought not, if placed at this spot, to be altered in its position. It 
ought to show perhaps a diminution of magnetic force transmitted through that spot ; 
but, for the reason before given (2868.), I conclude it would indicate a greater inten- 
sity, the increased power thrown upon it through the diminution of the conducting 
power of the air in that place causing it to act as a more powerful needle. 
2922. Proceeding to a point 6, there the lines of force have dip. The same physical 
effect will be produced upon them here as before, i. e. the portions in the atmosphere 
will open out ; but neither here nor in the former case will they continne to have the 
same curvature as before, for towards and in the earth, where they have their origin, 
they are restrained more or less from altering by the unchanging action of the earth 
(2919.) ; whilst at their more advanced parts, as at c, they enter into portions of the 
atmosphere which are nearer to the most intense lines of solar action, H C, probably 
also into the region of most intense action, and also into space, circumstances which 
cause more displacement of the lines, tending to separate by the tension of the parts 
altered in the air, than can happen in the earth (2848.). So the magnetic line of force 
at b will not move parallel to itself, but being inclined a certain degree to the horizon, 
when in the normal condition, will be more inclined, i. e. will have more dip given to 
it by the presence of the sun. This is the fact made manifest by the needle when in- 
dicating the position of the line as to inclination (2908.) at Flobarton, Toronto or 
elsewhere, by the motion of its upper end ; for it is manifest that whatever happens 
on one side of the place of the sun and magnetic equator, when, as in our supposition 
(2920.), they coincide, will happen on the other. 
MDCCCLI. K 
Fig. 14. 
H 
