Item 18 - Letter from George Airy

Observatory - GA gives his views regarding Barlow's [correction] of ship-magnetism: 'The importance of its error (other changes not considered) increases as the directive force of terrestrial magnetism on the horizontal needle decreases, that is as the dip increases. Under this circumstance, the absolute error is small, because the section perpendicular to the dip is nearly the same in all portions of the ship. So that the error is small in the circumstance in which its relative effect is great. On this account it is doubtless a good thing when most wanted' GA gives his breakdown of the 'facts' concerning the theory of humming tops.