It is a well-known fact that there are very few points on the earth's surface where the compass needle points either to the true geographical pole or to the magnetic pole, and if it does chance to do so, it is a transient happening. The "variation of the compass" or the declination of the needle, as it is called, is the angle that the compass needle makes with the true north and south line or the meridian. It is an angle of greatest importance to navigators and explorers, for it gives them their bearings, yet it is unfortunately subject to ceaseless variations of a most complicated nature, since it depends on the constantly pulsating and never ceasing magnetic changes that sweep over the surface of the earth and through its crust. It is affected by long period or secular changes, as they are called, progressing more or less regularly in obscure cycles of unknown period. It is subject to a diurnal change that depends on the position of the sun relative to the meridian, and that varies with the seasons and with the hour of the day. It is affected by the sun spot cycle of 11.3 years which has a direct effect upon the intensity of the earth's magnetic field. The intensity of the magnetic field in sun spots is, according to Abbot, sometimes as high as 4,500 gausses or 9,000 times the intensity of the earth's field. At times of maximum spottedness of the sun the intensity of the earth's magnetic field is reduced. Moreover, when great and rapidly changing spots appear upon the sun, electrified particles are shot forth from the sun with great velocity and in great numbers, and are drawn in towards the magnetic poles of the earth. Meeting the rarefied gases of the earth's upper atmosphere, they illuminate them as electric discharges illuminate a vacuum tube. Some of these electrons are absorbed by gases at high elevations, other descend to lower levels. The most penetrating rays have been known to descend to an altitude of twenty-five miles which is about the lowest limit yet found for auroral displays. It is the passage of these rays through the atmosphere that cause the magnetic disturbances known as _magnetic storms_, that are associated with the appearance of great sun spots and auroral displays. At such times sudden changes take place in the intensity of the earth's magnetic field that cause the compass needle to shiver and tremble and temporarily lose its directive value. These _magnetic storms_ have been known to produce great temporal changes in the intensity of the earth's field. According to Dr. L. A. Bauer, Director of the Department of Terrestrial Magnetism of the Carnegie Institute of Washington, the earth's intensity of magnetization was altered by about one-twentieth or one-thirtieth part by the magnetic storm of September 25, 1909, which was one of the most remarkable on record, and the earth's magnetic condition was below par for fully three months afterwards as a result.