ATMOSPHERIC MAGNETISM — EFFECT OF COOLED AIR. 
51 
cially those on the right and left, and even at h and h, where the power transmitted, 
instead of being more, as at P, is really less than the portion transmitted in the normal 
or equable state of the magnetic field. With a diamagnetic globe of air, i. e. one warmer 
or more rarefied than the surrounding space (2877-)? though it would convey less 
power as being a worse conductor, still it should cause the magnet to set with greater 
force, and so give an indication of increased intensity, and that also both within and 
equatorially without the globe. 
2873. If it be true that the changes of the medium (2869.) can thus affect the 
magnet, and that such changes can rise up to a sensible degree in the gases, then a 
magnet might make a different number of vibrations in a given time in oxygen and 
nitrogen gases of the same density, for they are very different in their magnetic rela- 
tions. It should make the greatest number in nitrogen; perhaps a delicate torsion 
balance would be a still more sensible test of such a result ; but it is probable that 
the space around the needle should be large, and it would be requisite to ascertain 
that the two media opposed equal mechanical resistance to the vibrating needle. 
2874. The variation of the direction caused by the typical globe (2864.) might be 
oblique to the horizontal and vertical planes, and consequently give results of de- 
clination and inclination, either separately or together. The direction would notvary 
in a central line parallel to the general dip of 
the surrounding space (fig. 7). Along another 
central line perpendicular to this {i. e. any line 
in the equatorial plane), a P, there would also 
be no variation of the direction, but in any 
other position there would be variations. Thus 
in the line i r, as the free needle passed from 
i to k, its lower end would be carried inwards 
towards the central line of dip P ; this effect, 
after attaining a maximum, perhaps at /, would 
gradually diminish again, and by the time the 
needle had reached r the dip would be normal. 
Coiresponding effects would occur on the opposite side of the axial line p e; and if 
a needle be considered as in any place the dip of which is thus affected, and then be 
conceived as travelling in a circle round the axial line p e, it would always be in the 
surface of a cone, the apex of which is below. 
2875. On the other hand, if the variations of the dip below the equatorial plane a P 
be considered, they will be equal in amount, but in the reverse direction, so that the 
magnetic needle, when deflected from its normal position, would have its upper end 
inclined inwards towards the axial line jo e; or if moved round the axial line would 
always be in a conical surface, the apex, of which is above. 
2876. So the dip would vary in such a globe of air in every azimuth ; and it would 
also vary in opposite directions in the upper and lower parts of the globe, and of the 
affected surrounding space. 
H 2 
Fig. 7. 
