consists of
five principal parts--the card, the needles, the bowl, a jewelled cap
and the pivot. The card or "fly," formerly made of cardboard, now
consists of a disk either of mica covered with paper or of paper alone,
but in all cases the card is divided into points and degrees as shown in
fig. 1. The outer margin is divided into degrees with 0 deg. at north
and south, and 90 deg. at east and west; the 32 points with half and
quarter points are seen immediately within the degrees. The north point
is marked with _fleur de lis_, and the principal points, N.E., E., S.E.,
&c., with their respective names, whilst the intermediate points in the
figure have also their names engraved for present information. The arc
contained between any two points is 11 deg. 15'. The mica card is
generally mounted on a brass framework, F F, with a brass cap, C, fitted
with a sapphire centre and carrying four magnetized needles, N, N, N, N,
as in fig. 2. The more modern form of card consists of a broad ring of
paper marked with degrees and points, as in fig. 1, attached to a frame
like that in fig. 3, where an outer aluminium ring, A A, is connected by
32 radial silk threads to a central disk of aluminium, in the centre of
which is a round hole designed to receive an aluminium cap with a highly
polished sapphire centre worked to the form of an open cone. To direct
the card eight short light needles, N N, are suspended by silk threads
from the outer ring. The magnetic axis of any system of needles must
exactly coincide with the axis passing through the north and south
points of the card. Single needles are never used, two being the least
number, and these so arranged that the moment of inertia about every
diameter of the card shall be the same. The combination of card, needles
and cap is generally termed "the card"; on the continent of Europe it is
called the "rose." The section of a compass bowl in fig. 4 shows the
mounting of a Thomson card on its pivot, which in common with the pivots
of most other compasses is made of brass, tipped with osmium-iridium,
which although very hard can be sharply pointed and does not corrode.
Fig. 4 shows the general arrangement of mounting all compass cards in
the bowl. In fig. 5 another form of compass called a liquid or spirit
compass is shown partly in section. The card nearly floats in a bowl
filled with distilled water, to which 35% of alcohol is added to prevent
freezing; the bowl is hermetically sealed wi