ATMOSPHERIC MAGNETISM — MAGNETIC STORMS. 
77 
results the conclusion, that there was a little diminution, and Professor Forbes, from 
his experiments made in different parts of Europe*, concludes that there is a de- 
crease of the force upwards. Such decrease may be a real consequence due to the 
difference of distance from the source of the terrestrial magnetic force ; or, as is more 
likely, it may be due to the different proportions of oxygen there and at the surface of 
the earth. According to Gay-Lussac’s account of the air brought from above, it was as 
0'5 to 1-0, compared with the density below. Hence the paramagnetic power, added 
to space in the place above, from whence the air was taken, would not be more than 
one-half of that added by the presence of the denser atmosphere below. This I think 
ought to make a change in the distribution of the magnetic force ; it would almost 
certainly do so at the equator, where the lines of force are parallel to the general 
direction of the atmosphere (2881.) ; and I think it would do so, as to the hoi-izontal 
component, in the latitude where Gay-Lussac and Biot made their aerial voyages. It 
is also just possible that the observers may have been in such relation to the heated 
or cooled air about them as to have had the difference observed produced, or rather 
affected, by some of the circumstances just described (2951.). 
2962. Whether the result obtained by Gay-Lussac and Biot indicate a change of 
power due to distance or not, this we know, that there are great changes from the 
magnetic equator toward the north and south ; and that, as Humboldt and Bessel 
say, it is doubled in proceeding from the equator to the western limits of Baffin’s Bay. 
And when so little as one-third of a cubic inch of oxygen can exert a force equal to 
the tenth of a grain, subject to the action of our powerful magnet, we may well con- 
ceive that the enormous sum of oxygen present, in only a few miles of heated or 
cooled atmosphere, can compensate for the great difference of magnetic force, and 
so, by a change of place, cause currents or winds having their origin in magnetic 
power. In such a case we should have a relation of magnets to storms ; and the mag- 
netic force of the earth would have to do with the mechanical adjustments and 
variations of the atmosphere, sometimes causing currents which without it might 
not exist, and at other times opposing those which might else arise, according as the 
great differential relations by which it would act (2757-) should combine with or 
oppose the other natural causes of motion in the air. Such movements would react 
upon the magnetic forces, so that these would readjust themselves, and so there 
would be magnetic storms, both material and potential, in the atmosphere, as there 
are supposed to be of the latter kind in the earth. 
2963. In bringing this communica,tion to a close, I have to express my obligations 
to two kind and able friends. Colonel Sabine and Professor Christie, for the interest 
they have taken in the subject, and on the part of the former for the extreme facilities 
afforded me in the use of observations and the data derived from them ; but in doing 
so I must be careful not to convey any idea that they are at all responsible for the 
peeuliar views I have ventured to put forth. I may well acknowledge that much 
* Edin. Phil. Trans., 1836, vol. xiv. p. 25. 
