The qualities of the terrestrial magnetism at any place as expressed in
its action upon the magnetic needle. Three data are involved.

I. The Declination or Variation.
II. The Inclination or Dip.
III. The Force or Intensity.

I. The Declination is the variation expressed in angular degrees of the
magnetic needle from the true north and south, or is the angle which the
plane of the magnetic meridian makes with that of the geographical
meridian. It is expressed as east or west variation according to the
position of the north pole; east when the north pole of the needle is to
the east of the true meridian, and vice versa. Declination is different
for different places; it is at present west in Europe and Africa, and
east in Asia and the greater part of North and South America. The
declination is subject to (a) secular, (b) annual and (c) diurnal
variations. These are classed as regular; others due to magnetic storms
are transitory and are classed as irregular, (a) Secular variations. The
following table shows the secular variations during some three hundred
years at Paris. These changes are termed secular, because they require
centuries for their completion.

Table of Declination or Variation at Paris.
Year.   Declination.
1580   11º 30′ E.
1663    0°
1700    8° 10′ W.
1780   19º 55′ W.
1785   22º 00′ W.
1805   22º  5′ W.
1814   22º 34′ W.
1825   22° 22′ W.
1830   22º 12′ W.
1835   22º  4′ W.
1850   20º 30′ W.
1855   19º 57′ W.
1860   19º 32′ W.
1865   18º 44′ W.
1875   17º 21′ W.
1878   17º 00′ W.
[Transcriber's note The value for 2008 is about  0° 48' W, changing by
0° 7' E/year.]

On scrutinizing these figures it will be seen that there is part of a
cycle represented and that the declination is slowly returning to the
zero point after having reached its maximum western variation in 1814.
Upwards of 300 years would be required for its completion on the basis
of what is known. In other places, notably the coast of Newfoundland,
the Gulf of the St. Lawrence and the rest of the North American seaboard
and in the British Channel, the secular variations are much more rapid
in progress. (b) Annual variations–These were first discovered in 1780
by Cassini. They represent a cycle of annual change of small extent,
from 15′ to 18′ only. In Paris and London the annual variation is
greatest about the vernal equinox, or March 21st, and diminishes for the
next three months, and slowly increases again during the nine following
months. It varies during different epochs. (c) Diurnal variations were
discovered in 1722 by Graham. A long needle has to be employed, or the
reflection of a ray of light, as in the reflecting galvanometer, has to
be used to observe them. In England the north pole of the magnetic
needle moves every day from east to west from sunrise until 1 or 2 P.
M.; it then tends towards the east and recovers its original position by
10 P. M. During the night the needle is almost stationary. As regards
range the mean amplitude of diurnal variations at Paris is from April to
September 13′ to 15′; for the other months from 8′ to 10′. On some days
it amounts to 25′ and sometimes is no more than 5′. The amplitude of
diurnal variations decreases from the poles to the equator. Irregular
variations accompany earthquakes, the aurora borealis and volcanic
eruptions. In Polar regions the auroral variations may be very great;
even at 40° latitude they may be 1° or 2°. Simultaneous irregularities
sometimes extend over large areas. Such are attributed to magnetic
storms. II. The Inclination is the angle which the magnetic needle makes
with the horizon, when the vertical plane in which the needle is assumed
to be free to move coincides with the magnetic meridian. It is sometimes
called the dip of the needle. It varies as does the declination, as
shown in the following table of inclinations of London.

Table of Inclination or Dip at London
Year.   Inclination.
1576   71° 50′
1600   72°
1676   73° 30′
1723   74° 42′
1773   72° 19′
1780   72°  8′
1790   71° 33′
1800   70° 35′
1821   70° 31′
1828   69° 47′
1838   69° 17′
1854   68° 31′
1859   68° 21′
1874   67° 43′
1876   67° 39′
1878   67° 36′
1880   67° 35′
1881   67° 35′

III. Force or Intensity is the directive force of the earth. It varies
with the squares of the number of oscillations the magnetic needle will
make if caused to oscillate from a determined initial range. The
intensity is supposed to be subject to secular change. According to
Gauss the total magnetic intensity of the earth is equal to that which
would be exerted if in each cubic yard there were eight bar magnets,
each weighing one pound. This is, of course, a rough way of expressing
the degree of intensity. Intensity is least near the magnetic equator
and greatest near the magnetic poles; the places of maximum intensity
are termed the magnetic foci. It varies with the time of day and
possibly with changes in altitude.