maximum effect, and the amount of deviation only would have been affected had the power of the iron in the ship been uniformly increased. Colonel Sabine ascribes this change in the amount of deviation to a different, and perhaps a more probable, cause; and as it is a point of some importance to determine, I will give his explanation in his own words, first remarking, however, that as the greatest care had been taken that the distribution of iron in the ship should be always the same, or as nearly so as possible, the deviation of the plane of no effect amounting to more than a point and a half, cannot have arisen from any slight modifications of this nature, but must be ascribed to some other cause. He observes[1]:—
"After the arrival of the expedition at Hobarton, and before it sailed to the Antarctic Circle, a similar series of observations was made in the Erebus, on the 29th of October, 1840, and again repeated on her return to Hobarton the following autumn, viz. on the 29th of June, 1841. The south end of the needle being now the one which dipped below the horizon (the dip being 70.40 S.), the deviation of the compass was found to take place in the contrary direction to that which had been observed at Gillingham, the disturbance being towards the west as the ship's head went round from north by east to south, and towards the east as her head passed from south through west to north.
- ↑ Phil. Trans. R.S. Part II. 1843, p. 152.
