The above terms are exceedingly near 864, 1246, 1728, 4320, and those numbers are equal to the mystic number of the Indians, Greeks, and Egyptians, 432 multiplied by 2, 3, 4, and 10. On these the ancients believed a certain combination of natural events to depend, and, according to Brahminical mythology, the duration of the world is divided into four periods, each of 432,000 years. Again, the sun’s mean distance from the earth is 216 radii of the sun, and the moon’s mean distance 216 radii of the moon, each the half of 432. Proceeding with this very curious examination, Hansteen says, 60 multiplied by 432 equals 15,920, the smallest number divisible at once by all the four periods of magnetic revolution, and hence the shortest time in which the four poles can complete a cycle, and return to their present state, and _which coincides exactly with the period in which the precession of the equinoxes will amount to a complete circle_, reckoning the precession at a degree in seventy-two years.[176] When we consider the phenomena of terrestrial magnetism carefully, it appears to indicate the action of a power external to the earth itself, and, as Hansteen conceives, having its origin from the action of the sun, heating, illuminating, and producing a magnetic tension, in the same manner as it produces electrical excitation and actino-chemical action. The movements of these magnetic poles have been the subject of extensive and most accurate observation in every quarter of the globe. In London, during 1657-1662, there was no magnetic variation; the agonic line passing through it. The variation steadily increased, until, in 1815, it amounted to 24° 15' 17", since which time it has been slowly diminishing. In addition to this great variation, we have a regular annual change dependent on the position of the sun, in reference to the equinoctial and solstitial points, which was discovered by Cassini, and investigated by Arago and others. Also a diurnal variation, which movement appears to commence early in the morning, moving eastward until half-past seven, A.M., when it begins to move westward until two, P.M., when it again returns to the east, and in the course of the night reaches the point from which it started twenty-four hours before. We have also remarkable variations in what is termed the _dip_ of the needle. It is well known that a piece of unmagnetized steel, if carefully suspended by its centre, will swing in a perfectly horizontal position, but, if we magnetize this bar, it will immediately be drawn downwards at one end. The force of the earth’s polarity, attracting the dissimilar pole, has caused it to _dip_.