Art. XV.—On Terrestrial Magnetism.

Transactions and Proceedings of the Royal Society of New Zealand, Volume 22, 1889, Page 129

Art. XV.—On Terrestrial Magnetism. By the Hon. Robert Hart. [Read before the Wellington Philosophical Society, 19th February, 1890.] Observations on the subject have determined that the magnetism of the earth is that of a hollow sphere. The problem here proposed for solution is the constitution of that hollow sphere. The earth, an oblate spheroidal mass, having a diameter at the equator of about eight thousand miles, travelling through space at the speed of about nineteen miles in a second of time, is held in its place toward the sun by the centripetal and centrifugal action of its gravity in motion. While so travelling and so kept in its place the earth revolves upon its axis in rather less than twenty-four hours, carrying with it on its surface an elastic cushion, called its atmosphere. This atmosphere consists of air containing oxygen and hydrogen gases interfused with a trace of a third. Observations on the aberration of light have determined the depth of this atmosphere from its outer surface to be between forty-five and fifty miles. That to the depth of forty miles from its outer surface this atmosphere is absolutely dry must appear from the fact that water in a state of vapour would at that distance from the surface of the earth lose the heat needful to keeping it in that state. Even at his place of observa-

tion on the Peak of Teneriffe, Piazzi Smith found the desiccating power of the atmosphere such that the ink he used in writing would often dry in the pen during its passage from the inkstand to the paper; and those of his instrument-boxes which were fastened with glue came in pieces, from the moisture necessary to its holding-power being dried out of the glue, and the glue converted into powder. That dry air is a non-conductor of electricity was proved fifty years ago at an exhibition in what was called the Adelaide Gallery, in London, where the discharge of the electricity stored in an enormous Leyden jar through a column of dry air in a glass tube caused it to pass as a ball of white light, occupying an appreciable time in its passage; while a similar discharge through a similar tube from which the air had been withdrawn poured as a stream of purplish light. This proves that dry air offers. pro tanto, obstruction to the passage of electricity, which the absence of air freely permitted. Dry air, being a non-conductor of electricity, must, in the same way as glass, sealing-wax, and dry brown paper—non-conductors of electricity—be a good electric, and, subjected to appropriate conditions, generate electricity. Faraday demonstrated that oxygen gas is magnetic—that is, acted upon by electricity it acquired polarity in the same sense as steel does. This cushion of electric matter carried round the earth in its diurnal revolution cuts the lines of physical force which hold the earth in its place toward the sun while travelling through space at rates varying from a thousand miles an hour to nothing. That a process analogous to friction must be the result is a reasonable conclusion, deriving confirmation from the existence of the trade-winds and the Gulf Stream. That the earth's atmosphere, revolving at such speed, and exposed to the action of those actively-acting lines of physical force, could be so exposed without a physical effect being produced, is a conclusion which I cannot believe. This effect upon the dry atmosphere is the generation of electricity, which, acting upon its oxygen, gives magnetic force directly to so much of the oxygen as is directly subject to the electrical action, and by induction to any not so subject. And thus is constituted the hollow sphere to which are due the phenomena of terrestrial magnetism. The view here put forward is confirmed by the coincidence observed between the appearances of spots on the sun and those of the aurora borealis and aurora australis on the earth. Spots on the sun are produced by the rolling-away from over large surfaces of that luminary of the cloudy envelope, his photosphere. The effect of such rolling-away is to intensify the action of the sun's gravity upon the earth's

atmosphere and to abnormally excite the magnetism of its oxygen. This abnormally - magnetized oxygen, in the usual circulation of the air occasioned by the heat of the tropics flowing over towards the poles and becoming condensed, discharges its abnormal electro-magnetism in light. The fact of the earth's atmosphere being thus rendered self-luminous suggests the probability that the luminosity of the sun is due to the friction of the planets rolling round him acting upon suitable materials. I am persuaded that, when opportunities of observation have been afforded, and the facts are ascertained, it will be found that the magnetic poles of the earth are not fixed in position, but movable. They are nearest to the axial poles of the earth at the times of the equinoxes, and farthest from them at the times of the solstices. The electro-magnetism of the earth's atmosphere on trees, plants, and animals has a vitalising influence. Upon trees and plants it operates by aiding the circulation of the sap. In man it influences the animal spirits. This can be proved by observation. The southerly breeze in this City of Wellington brings its oxygen into a normal electro-magnetic state. The breeze coming from the north in its passage over trees and plants has parted with some of its electro-magnetism, and is not so refreshing. Still more noticeable is the effect on a day of cloud and fine rain—for then trees and plants, rendered by the moisture which clings to them more than usually good conductors of electricity, are drawing from the atmosphere its electro-magnetism and almost visibly growing, while man and other animals are dull, and, as far as they are liable to be affected by weather, dispirited. In high latitudes the strength of men is not only due to the warm clothing and oleaginous food the consumption of which helps to keep up the animal heat, together with the action of the oxygen of the atmosphere upon the blood, but is also to be attributed to the electro-magnetism of that oxygen, which, while the air at a temperature many degrees below freezing-point is being inhaled, acts upon the nervous system, quickens the circulation of the blood, and vitalises the system.