The magnetic elements are also subject to changes, which have a yearly and a daily period. In describing these shortly, we shall limit ourselves to the changes affecting declination, as these are of most general interest. The following are the chief particu lars of the annual variation of declination given by Cassini: From April to July, or from the vernal equinox to the summer solstice, the western declination decreases. From the summer solstice to the vernal equinox, that is, during the other nine months of the year, the declination increases, the needle turning to the west. Its position in May and in October is nearly the same; so that in the winter months, from October to April, the westerly motion is slow. The range of the annual variation at Kew is 58'85".
The mean diurnal variation for Kew is shown in fig. 2. The irregular line indicates the course of the n. end of the needle. A rise of this line indicates a change of the n. end to the e. ; a fall, a change to the west. The interval between two horizontal lines corresponds to a deflection of the needle 1' to the e., and a fall 1' to the west. The line marked o is the magnetic meridian, or the mean daily position of the needle. The inter val between two upright lines corresponds to an hour. The course begins at twelve at night, and ends at twelve the following night. At twelve at night, the magnet is e. of the mean position, and continues nearly in the same position, with only a slight westerly deviation, till 15 hours (three in the morning), when it veers eastward. At 20 hours (eight in the morning), it reaches its furthest e. point. From eight in the morning till oue in the afternoon, it makes a sweep of 10' toward the w., and then stands about 6' to the w. of the mean. After one, it goes westward till midnight, when it again begins the same course. The needle stands in its mean position a little after ten in the morning, and a little before seven in the evening. The course here described is the course for the year. But the diurnal range is different in different months. In May, for instance, the average range between the extreme points is 12', which is the maximum range for the year; and in December, when it is a minimum, it is only 5' 28". The diurnal changes here described for Kew are much the same all over the n. magnetic hemisphere. The amount, however, is different. Near the magnetic equator the diurnal variation is little or nothing, and it increases as we go northward. Captain Duperrey states that at or
near the magnetic equator, the n. point of the needle in the morning shifts slightly e. or w. of the mean, according as the sun passes s. or n. of the station. In the southern magnetic hemisphere, the daily motions of the needle takes place much in the same way as in the northern hemisphere, only the s. pole take the place of the n. pole, and the direction of the deffiTtions is reversed. The correspondence, and at the same time oppo sition, of the southern hemisphere is also shown from the time of maximum and mini mum range. When the sun is in the northern signs of the zodiac, the range is a maxi mum in the northern, and a minimum in the southern hemisphere; and when the sun is in the southern signs, the reverse takes place. The diurnal variation is so small, that the ordinary compass-needle is not delicate enough to show it.
The irregular variations arc those which break in upon the regular march of the diurnal variation, without in the main altering it. Instead, for instance, of the needle steadily going westward from 8 A.M. CO 1 p.m., as shown in fig. 2, it makes, when sfeected by irregular variation, deflections eastward as well as westward, although it in the main moves westward; so that the line between these hours, instead of being com paratively straight, would be an irregular zigzag. These disturbances of the mean course are sometimes considerable, amounting even to one or two degrees in extreme cases. On some days, the mean diurnal course is much disturbed. on others, very little; but it is never quite free from them. It has been found that places of the same longi tude have similar disturbances at the same time; that those on opposite sides of the globe, or differing by 180° of longitude, have disturbances equal in amount, but oppo site in direction; and that those situated 90° w. or e. of the disturbed regions, have little or no disturbance. The appearance of auroras is invariably accompanied by magnetic irregularities, and their effect extends far beyond the regions where they are visible. Earthquakes and volcanic eruptions have also a marked effect in this way. Humboldt gave the name of magnetic storms to these irregular disturbances. The frequency of these storms, and the amount of the diurnal variation, are found to be the greatest when sun-spots are most numerous. See SUN.