not all of those where great differences had been observed, but also that the differences themselves were conformable to what had taken place upon the binnacle of the Investigator.
Mr. Wales goes on to observe, "It is not necessary to account for these differences in the observed variations in this place, nor yet to point out the reasons why such anomalies have not been noticed in observations of this kind before. I shall however remark, that I have hinted at some of the causes in my introduction to the observations which were made in captain Cook's second voyage; and many others will readily offer themselves to persons who have had much practice in making these observations, and who have attentively considered the principles upon which the instruments are constructed, and the manner in which they are fabricated. Nor is it at all surprising, that the errors to which the instruments and observations of this kind are liable, should not have been discovered before; since no navigators before us ever gave the same opportunity, by multiplying their observations, and making them under such a variety of circumstances as we did."
That the compasses, even in the Royal Navy and to this day, are the worst constructed instruments of any carried to sea, and often kept in a way to deteriorate, rather than to improve their magnetism, cannot be denied; but errors arising from the badness of compasses would not be reducible to regular laws as those were in the Investigator, and appear to be in the three ships commanded by captain Cook. It seems indeed extraordinary, that with the attention paid by Mr. Wales to the subject, he should not have discovered, or suspected, that the attraction of the iron in the ship was the primary and general cause of the differences so frequently observed; nor have perceived that the differences varied proportionally to the direction of the ship's head and to the dip of the needle, and were of an opposite nature in the two hemispheres. But it should be recollected, that the apparently contradictory phenomena which occur in most branches of science, frequently bewilder the inquirer in a labyrinth where pursuit seems to be hopeless; and that when one general cause is found to explain all the contradictions, to have hit upon the clue appears so easy that any one might have perceived it: the inquirer himself is not less surprised that it should have escaped him so long, than pleased at his final success.
