Previous to Captain Back’s departure in 1833 with the expedition for the relief of Captain Ross, he consulted me respecting the nature of the magnetical observations which I considered it desirable should be made in the regions he was likely to visit. I was fully sensible that, however available the expedition on which he had so nobly volunteered might be made to the cause of science, its primary object, and that to which all others must give place, was the relief of our gallant countrymen, at that time considered to be in imminent danger of perishing in the inhospitable regions which their enterprising spirit had led them to explore. I therefore considered it an object of the first importance, that whatever observations were to be made during the movements of the expedition should be so conducted as to cause as little delay as possible consistently with obtaining data for correct results, and also, that they should be made in the order of their importance. Compared with observations of the direction of the magnetic needle, both with reference to the meridian and to the vertical, other observations are of minor importance towards establishing anything like a theory of terrestrial magnetism. Considering that observations of the direction of the needle with reference to the meridian, though quite as important in a theoretical point of view as those with regard to the vertical, were necessarily called for in the conducting of the expedition, I, in the first instance, pointed out the observations which I considered necessary for determining the dip of the needle at the various stations where it might be practicable to make such observations; and I left the less important ones for the determination of the relative intensities of the absolute force acting upon the needle, which required more care, attention and assistance, to be made or not, according to the circumstances under which the expedition might be placed. Immediately on his return, Captain Back did me the favour to place all his magnetical observations at my disposal, and I feel that I should not do justice to the zeal and ability which that enterprising officer has displayed in the cause of science, if I did not, now that I have had leisure to reduce the observations, lay an account of them before the Royal Society.
Theoretical methods for the determination of mixing
