remarked, does not point due north and south, neither, when poised
freely on its centre, does it lie perfectly horizontal; in our latitude
it points at present 20° west of north, which is called its
_declination_, and its north pole slopes downwards at an angle of 68°,
which is called its _dip_.

By holding a rod of iron, or a poker, for a length of time parallel to
the direction of the needle, so as to have the same declination and the
same dip, it will gradually assume and display magnetic virtue, and this
will ere long become fixed and powerful under a succession of vibratory
shocks. There is a beautiful experiment in which a needle, when
magnetised, can be made to float on water, when it adjusts itself to the
magnetic meridian, and will incline north and south the same as the
needle of the compass.

_The Chemical Action of Electricity and Magnetism_.--These agents
possess powers which develop wonderfully in connection with chemical
combination. Thus, if we suspend a piece of iron in a vessel which
contains oxygen gas, and apply to the metal an electric current, it will
immediately begin to unite rapidly, and form an oxide with oxygen,
emitting, during the process, intense heat and a bright flame. Zinc,
too, when similarly acted on, will ignite in the common atmosphere and
burn away, though with less intensity, till it also is, under the
electric force, reduced to an oxide. It is presumed that many other
chemical combinations take place because of the simultaneous joint
development of electric agencies, as in copper, water, and aquafortis,
nitrate of copper, &c. So also it happens that, when a plate of iron is
for some time immersed in a copper solution, it comes out at length
covered over with a coating of copper. And it is because there is
electricity at work that a silver basin will be coated with copper when