bation produced exactly similar and synchronous effects on instruments placed at the respective distances of six hundred, and fifteen hundred miles, and which we had only two opportunities last year of observing, at a distance of about a thousand miles from Van Diemen's Land.
The iron tanks, chain cables, anchors, and all iron materials which had necessarily been removed during the repairs of the vessels, having been replaced in the exact spot from which they had been taken, the process of swinging the ship round, to redetermine their united effect upon the compass, was accomplished on the 29th of June. We were surprised to find that both in amount and direction it had very considerably altered. Thus the points of no effect had changed since October, 1840, from nearly N. by W. to nearly N. by E., and from nearly S. by E. to nearly S. by W.; and the amount and direction of extreme deviation from 4º 6′ with the ship's head E. by N. to 5º 30′ with her head E.S.E., and from 4º 16′ with her head W.S.W. to 5º 13′ with her head West.
These results point out in a striking manner the necessity of frequently repeating experiments of this nature, where an ordinary amount of accuracy is desirable; as they moreover serve to prove that some kinds of iron, and perhaps various positions in which it may be placed with reference to the line of dip, render them more susceptible of change than others, or no alteration could have occurred in the direction of the points of minimum and