Page:A Voyage of Discovery and Research in the Southern and Antarctic Regions Vol 2.djvu/44

1841 arrangement and distribution of the ship's iron, and an easy and natural explanation might appear to be afforded. It is, however, one of several facts which have presented themselves in the course of a careful examination of the observations of the first two years of the expedition, which seem to point to the possibility of a somewhat different cause; viz., that when a ship changes her magnetic latitude, the corresponding change in the magnetism of the ship, or, more strictly, in that portion of it which is derived from induction, follows, but does not always, or altogether, take place instantaneously. It would accord with this supposition, that the disturbance of the compass should be less in the Erebus on her first arrival at Hobarton in 1840, than on her return there in 1841; because in 1840 she had recently passed through the lowest magnetic latitudes, and in 1841 she came immediately from the highest. The observations in 1840 give a less value for α tan θ^[1], than those of 1841; and taking the dip at Hobarton as the value of θ, to which the induced magnetism of the ship on both occasions should strictly correspond, we should have a less value for α in 1840 than in 1841; whereas, if with the same dip we take a mean between the disturbances of the compass on the first arrival and on the return, by which we may be conceived to neutralise in a great measure the temporary influences which

1. See Phil. Trans. R.S. p. 149.