CXVlll 
APPENDIX. 
OBSERVATIONS 
ON THE VARIATION, AND ON THE IRREGULARITIES IN THE DIRECTION OP THE 
NEEDLE, OCCASIONED BY THE ATTRACTION OF THE SHIp’s IRON ; 
MADE ON BOARD II. M. S. THE HECLA, 1819—1820. 
In the course of these observations, two objects were designed to be 
kept in view ; it was requisite, first, for the purposes of navigation, 
that the amount of the variation on the courses steered by the ship should 
be known, that her true direction might be deduced from that indicated by 
the compass ; and secondly, to these necessary observations, it was desired 
to add such as could be made without material inconvenience or delay with 
the ship’s head placed on other points than those of her immediate courses, 
for the purpose of exemplifying more extensively than had been done 
heretofore, the irregularities which take place in the direction of compass 
needles, in consequence of the attraction of the iron contained in ships. 
Experience in the voyage of the Isabella and Alexander in 1818 , had 
shewn the necessity in high magnetic latitudes of adopting an expedient 
originally suggested by Captain Flinders, vk., the selection of some one 
spot in the ship as the permanent position of a standard compass, in which 
it should be invariably placed for use, whether in observing azimuths, or 
bearings of land, or in directing the ship’s course ; and that if on any 
particular occasion, it should be necessary to use a compass in any other 
part of the ship, a reference should be made to the standard by comparison, 
and the difference (if any) in its pointing noted and allowed for ; a certain 
degree of uniformity being found to obtain in the effects of the local 
attraction on a compass thus confined to one spot, enabling a navigator 
by occasional observations and the aid of such practical rules as former ex- 
periments had suggested, to form a sufficiently correct judgment of the 
different amounts of variation to be allowed with it on each change in the 
direction of the ship’s head. It was considered, that a series of observa- 
tions, shewing the uniform nature of these effects on a standard compass 
might prove of service, in confirming and giving confidence to the inferences 
which had been drawn from former experiments, and might possibly conduce 
towards an improvement of the rules which had been founded on their results. 
Moreover, as it was known that the amount of irregularity on each and 
