h is
directed toward the north) moves to the east from the night until between
seven and eight o'clock in the morning, when an opposite movement
commences, and the north end of the magnet moves to the west. Recent
observations have shown that a similar movement takes place at the same
hours of local time in North America, and that it is general in the middle
latitudes of the northern hemisphere; but to show the capricious nature of
magnetism, it may be mentioned, that although in the southern portion of
the globe the movement of the magnet in the contrary direction is constant
throughout the year, yet at St. Helena the peculiar feature of the diurnal
is, that during one half of the year the movement of the north end of the
magnet corresponds in direction with the movement which is taking place in
the northern hemisphere, while in the other half of the year the direction
corresponds with that which is taking place in the southern hemisphere.

Another striking result of these investigations is the estimate of the
total magnetic power of the earth as compared with a steel bar magnetized
one pound in weight. This proportion is calculated as
8,464,000,000,000,000,000,000 to 1, which, supposing the magnetic force
uniformly distributed, will be found to amount to about six such bars to
every cubic yard of the earth's surface.

Thus measured, it will be seen how tremendously mysterious is the power of
magnetism, and how potent an influence it must possess over animate and
inanimate nature! And not one of its least wonderful mysteries is its
singular exception to the character of stability and permanence. The
configuration of our globe, the distribution of temperature in its
interior, the tides and currents of the ocean, the general course of
winds, and the affections of climate--all these are appreciably constant.
But magnetism, that subtle, undefinable fluid, is perpetually undergoing a
change, and of so rapid a nature, that it becomes necessary to assume
epochs, which ought not to be more than ten years apart, to which every
observation should be reduced. The extreme importance of knowing the exact
amount of magnetic variation can scarcely be overrated for maritime
purposes; and the establishment of a complete magnetical theory, based on
an extensive series of observations, must be regarded as a desideratum by
the first nautical country.

The numerous magnetical surveys that have been made by our government,
taken in conjun