Some philosophers have supposed that earthquakes are accompanied by magnetic disturbances; but the great Humboldt doubts this, though he was startled by observing, during the violent earthquake of Cumana in 1799, "that the dip was diminished by 48'." Magnetic phenomena appear to be immediately connected with meteorological changes. Hence it is that meteorology forms an important part of the work of a magnetic observatory; and observations of the thermometer, barometer, wind, and weather are made simul taneously with those of the magnets. Violent gales, sudden changes of temperature, and rapid barometric fluctuations are generally accom panied by magnetic disturbances.
Each place on the earth's surface has its own distinct magnetic elements. We have already stated that the freely suspended needle does not point exactly north and south, but to two points near the terrestrial poles. These points are called the magnetic poles. I When the meridian of any place likewise passes through the magnetic poles, it is evident that the magnetic meridian coincides with the terrestrial, and there is no declination or variation ; and the locus of all such places is called " the line of no variation." Again, at places near the equator, the dip needle is horizontal, and the locus of all such places, which locus does not coincide with the equator, nor is it an exact circle, is called the "line of no dip." As we proceed towards either of the terrestrial poles, the needle becomes more and more inclined to the horizon, until we reach the magnetic poles, where it assumes a vertical position.
Likewise, also, the absolute intensity of the magnetic force depends upon locality. It appears to increase as we proceed from the equator to the poles, and the curves which pass through all those places where the intensity is the same, are called " isodynamic lines." These lines do not coincide exactly with parallels of latitude, nor with lines of equa? dip. According to the most recent determination, the north magnetic pole is situated very nearly in latitude 5', and west longitudo Some have supposed the existence of two magnetic poles in each hemi sphere, the position of the second northern polo being in Siberia, about north Latitude, and east longitude, The positions which have ieen assigned to the southern poles are a little to the south of Australia, .d south-south-east of New Zealand. This hypothesis, however, must
re adopted with caution.
The following is a comparative view of the value of the magnetic dements at some places where they have been determined with the ;reatest accuracy The causes of terrestrial magnetism have still to be developed. Various theories have been from time to time proposed, some of which cave indeed successfully accounted for a few of the known magnet [cal phenomena, but fail under the tests which science and obser vation supply us with. The elaborate theory founded ou the Supposition that the earth is an actual magnet, is overturned by the discoveries of Faraday, Arago, and other philosophers. Although there does not appear to be a simultaneous occurrence of and electric storms, yet there can be little doubt of the connection between these phenomena, even if magnetism be not "one of the numerous forms under which electricity developes itself." The experiments of Faraday and Oersted have shown, not only that " electricity induces magnetism in the vicinity of the body which conducts it," but that " free magnetism gives rise to electricity." Assuming, what is highly probable, that the interior of the earth is a mass of liquid fire, then the idea of a magnetic nucleus in tho earth must be abandoned, for it is found by experiment, that the magnetism of a body is de stroyed when it reaches a white heat. It is, therefore, the earth's crust only which we may assume to be the seat of magnetic currents. That such currents do exist, is by no means improbable, and experi ments seem to show that they arcimmediately excited by unequal distribution of heat. If we regard the aurora borealis [TERRESTRIAL Ltoirr] as a luminous dischaige of superabundant magnetism, we can understand how this phenomenon should be confined to the vicinity of the colder regions, where the earth's surface is least heated, and the magnetic intensity, therefore, the greatest. No less than 146 years ago, Halley e Philosophical Transactions') had boldly ventured to con jecture, that the terrestrial light was a magnetic phenomenon. But when the fact became established, that the aurora indicated its appearance by the disturbed state of the magnet in all parts of the globe, and the brilliant discovery was made by Faraday, that light could be evolved by magnetic power, then a passing conjecture ac quired a degree of probability amounting almost to certainty.