ATMOSPHERIC MAGNETISM — DIURNAL VARIATION — DIRECTION. 
67 
in still higher latitudes have, as before their inclination, increased. On the other side 
of the equator, the tendency of the lines woidd be to increase in inclination. 
2929. Proceeding to that part of the expected change of position of the free needle 
which produces variations of declination, let e r in fig. 15 re- 
present the sun’s path in the equator, and t c, t' c' the same 
at the tropics ; let r be a magnetic meridian, and a a', i i', o o' 
places of equal north and south inclination on opposite sides 
of the equator. The curves of magnetic force seen in front in 
fig. 14, are now in the plane of the magnetic meridian, but 
may be considered as rising on opposite sides of the equator 
and coalescing over it. If the air on all sides were in its mean 
condition and the sun entirely away, these curves would be 
in the vertical plane mr; or if the sun near midday was so 
placed that the resultant of the heated and changed atmosphere was in the meridian 
m r, though effects of inclination would occur (2922.), still the curves would remain 
in the same vertical plane. But if the resultant were either to the east or the west of 
m r, variations of declination would be produced. For suppose the sun to be advancing 
from the east or r ; because it gives the air a diamagnetic condition, the lines of force 
would tend to expand {2877-), and therefore move westward, as represented in the 
meridian ns; and the deflection caused thereby would be greatest upon the surface 
of the earth, beeause it is there that the curves as they enter the earth are held and 
restrained in respect of their normal position (2919.). As the vvarmed atmosphere 
came on, the western deflection would increase to a certain extent, and then diminish 
to nothing when the resultant was in the meridian ; but as the latter passed on, the 
deflection would grow up on the eastern side of ns, and, after attaining a maximum, 
would diminish and cease as the warm air retreated. 
2930. If the sun’s path was in the northern tropic, f c, and the resultant in the at- 
mosphere therefore to the north of the stations a or i, though that would make a 
difference in the amount of the declination variation, it would not alter its direction, 
for still the curves a d and i i' would bear to the west as the sun came up, and would 
be on the meridian when the resultant was there also. There would be more effect 
produced at i than at i', but the contrary character of the dip, in respect of the sun’s 
place, would not alter the direction of the declination variation. 
2931. A cold region of air acting, as at the coming on of night, upon the lines of 
magnetic force of the earth, would, by virtue of its paramagnetic character (2865.), 
produee corresponding effects both of inclination and declination, but in the contrary 
direction. 
2932. Thus the lines of force which issue from the earth at all places upon its sur- 
face where there is any dip, will, by the hypothesis, under the daily influence of the 
sun, describe by their aseending parts a closed curve or irregular cone, the apex of 
whieh is below. As a fact this result is perfeetly well known, but its accordance with 
the hypothesis is important for the latter. The mean position of the free needle will 
K 2 
Fig. 15. 
n 
7n 
0 
0 
c 
/ 
e 
l‘ 
r 
t 
a 
r 
1 
L 
1 
c 
^0' 
p r 
0 
