ber,
and the second on the 19th of November, 1850. In these memoirs he
discusses the effects of heat and cold upon the magnetism of the air,
and the action on the magnetic needle, which must result from thermal
changes. By the convergence and divergence of the lines of terrestrial
magnetic force, he shows how the distribution of magnetism, in
the earth's atmosphere, is effected. He applies his results to the
explanation of the Annual and of the Diurnal Variation: he also
considers irregular variations, including the action of magnetic storms.
He discusses, at length, the observations at St. Petersburg, Greenwich,
Hobarton, St. Helena, Toronto, and the Cape of Good Hope; believing
that the facts, revealed by his experiments, furnish the key to the
variations observed at all these places.

In the year 1851, I had the honour of an interview with Humboldt, in
Berlin, and his parting words to me then were, 'Tell Faraday that I
entirely agree with him, and that he has, in my opinion, completely
explained the variation of the declination.' Eminent men have since
informed me that Humboldt was hasty in expressing this opinion. In
fact, Faraday's memoirs on atmospheric magnetism lost much of their
force--perhaps too much--through the important discovery of the relation
of the variation of the declination to the number of the solar spots.
But I agree with him and M. Edmond Becquerel, who worked independently
at this subject, in thinking, that a body so magnetic as oxygen,
swathing the earth, and subject to variations of temperature, diurnal
and annual, must affect the manifestations of terrestrial magnetism.[1]
The air that stands upon a single square foot of the earth's surface
is, according to Faraday, equivalent in magnetic force to 8160 lbs.
of crystallized protosulphate of iron. Such a substance cannot be
absolutely neutral as regards the deportment of the magnetic needle. But
Faraday's writings on this subject are so voluminous, and the theoretic
points are so novel and intricate, that I shall postpone the complete
analysis of these researches to a time when I can lay hold of them more
completely than my other duties allow me to do now.

Footnote to Chapter 12

[1] This persuasion has been greatly strengthened by the
recent perusal of a paper by Mr. Baxendell.

Chapter 13.

Speculations: nature of matter: lines of force

The scientific picture of Faraday would not be complete without a
referenc