[173] Cavallo, _On Magnetism_.--Cavallo was the first who noticed the influence of heat on Magnetism. Consult _On the anomalous Magnetic Action of Hot Iron between the white and blood-red heat_: by Peter Barlow, Esq.--Philosophical Transactions, 1822, p. 124. _Treatise on Magnetism_: by Barlow.--Encyclopædia Metropolitana. [174] “The foundation of our researches is the assumption that the terrestrial magnetic force is the collective action of all the magnetised particles of the earth’s mass. We represent to ourselves magnetisation as the separation of the magnetic fluids. Admitting the representation, the mode of action of the fluids (repulsion of similar, and attraction of dissimilar, particles inversely as the square of the distance) belongs to the number of established truths. No alteration in the results would be caused by changing this mode of representation for that of Ampère, whereby, instead of magnetic fluids, magnetism is held to consist in constant galvanic currents in the minutest particles of bodies. Nor would it occasion a difference if the terrestrial magnetism were ascribed to a mixed origin, as proceeding partly from the separation of the magnetic fluids in the earth, and partly from galvanic currents, in the same; inasmuch as it is known that for each galvanic current may be substituted such a given distribution of the magnetic fluids in a surface bounded by the current, as would exercise in each point of external space precisely the same magnetic action as would be produced by the galvanic current itself.”--_General Theory of Terrestrial Magnetism_, by Professor Carl Friedrich Gauss, of the University of Göttingen.--Scientific Memoirs, vol. ii. p. 188. [175] Humboldt’s _Cosmos_.--Otté’s translation. [176] Hansteen: _Untersuchungen über den Magnetismus der Erde_, Christïana, 1819. Humboldt: _Exposé des Variations Magnétiques_.--Gilbert’s Annales. Brewster’s Magnetism: Encyclopædia Metropolitana. [177] Hansteen; as above. [178] _On the effects of temperature on the intensity of magnetic forces, and on the diurnal variations of the terrestrial magnetic intensity_; by Samuel Hunter Christie, Esq.--Philosophical Transactions, vol. cxv. 1825. [179] It has been observed by Mr. Barlow, in England, and some eminent observers in Austria, that an electric current constantly traverses the wires of the electric telegraph wherever there are two earth connections. [180] _Meteorological Observations and Essays_: by Dr. Dalton. _On the Height of the Aurora Borealis above the surface of the Earth_: by John Dalton, F.R.S.--Philosophical Transactions, vol. cxiv. p. 291. [181] Arago: Annales de Chimie, vol. xxxix. p. 369. _On the variable Intensity of Terrestrial Magnetism and the Influence of the Aurora Borealis upon it_; by Robert Were Fox.--Philosophical Transactions, 1831, p. 199.