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.