622 
APPENDIX. 
[Errors in variation. 
motion ; and it is only by taking the medium of several differences, that 
any thing like an accurate comparison can be established. 
After this exposition of the errors produced in the Investigator’s com- 
passes by the attraction of the iron, and of the method employed to obviate 
their effects on the survey, it will be asked whether any thing similar has 
been found in other ships'^ especially in those sent on discovery ; and if so, 
whether their observed variations and survey bearings were submitted to 
any regular system of correction ? It does appear that similar errors have 
been noticed in ships employed on discovery, as also in others, and that 
they probably exist in all ships, in a greater or less degree ; but as they 
were not perceived to follow any regular laws, no correction had hitherto 
been applied ; and it naturally follows, that there should be frequent discor- 
dance between the bearings given in Captain Cook’s voyages and others, 
and the charts which accompany them. There are few experienced sea- 
men who have not remarked occasional differences in the compass ; but the 
most general result of their observations seems to have been an opinion, 
that within some undefined and variable limits this instrument was radically 
imperfect ; and it has been not unusual, when an observed variation dif- 
fered much from what was thought to be the truth, to reject it, as having 
been either erroneously taken or bad from some unknown cause, and it is 
not entered in the journal. To this injudicious practice, than which nothing 
can more tend to stifle inquiry, and consequently prevent the advancement 
of knowledge, there are however many honourable exceptions ; and at the 
head of these must be placed the immortal Cook. In the introduction to 
the Astronomical Observations made in his second voyage, page 49, is the 
following passage from the pen of Mr. Wales, astronomer on board the 
Resolution. 
“ In the Channel of England, the extremes of the observed variations 
“ were from 19-f to 25° : and all the way from England to the Cape of 
Good Hope, I frequently observed differences nearly as great, without 
being aide, any way, to account for them ; the difference in situation 
being by no means sufficient. These irregularities continued after leav- 
ing the Cape, which, at length, put me on examining into the circum- 
stances under which they were made. In this examination it soon apw 
peared, that when most of those observations were made, wherein the 
