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 compasses 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 discordance between the bearings given in Captain Cook's voyages and others, and the charts which accompany them. There are few experienced seamen 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 differed 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 19½° to 25°: and all the way from England to the Cape of Good Hope, I frequently observed differences nearly as great, without being able, any way, to account for them; the difference in situation being by no means sufficient. These irregularities continued after leaving the Cape, which, at length, put me on examining into the circumstances under which they were made. In this examination it soon appeared, that when most of those observations were made, wherein the