||A magnetic dry-card compass of a type patented by Henry
Jennings in 1818 (no. 4259). The compass bowl is made of brass.
The card is mica covered with paper, and is marked in points,
with a fleur-de-lys at north. The unusual graduations resemble
those which used to be pasted into the bottoms of bowls when
just a needle was suspended on the pivot. The bowl has no
gimbals, although the patent drawings show two gimbal rings with
four planes of gimballing. The compass is now held in a wooden,
canvas-covered case with a leather strap (now broken). The
innovation of the design, Jennings claimed, was that its needle
was insulated ‘from all action by iron in its neighbourhood, to
which all other compasses are liable’. By this time, compasses
were well known to be influenced by any iron placed near them,
causing what is known as magnetic deviation, which causes errors
in the heading indicated by a compass. In Jennings' design, this
protection was provided by a layer of iron filings placed
between the inner and outer