Note 118 — Chapter II

See a learned and able report on the “Deviations of the Compass,” by Mr. Frederick J. Evans, Master R.N., Superintendent of the Compass Department of H.M. Navy, printed in the “Philosophical Transactions,” Part II. 1860. In this interesting paper, Mr. Evans calls attention to one or two important facts, certainly not known to the general public, or perhaps not even to many shipbuilders. He says, p. 354:

“In an iron sailing-ship, built head to south, there will be an attraction of the north point of the compass to the head, and if built head to north, a like attraction to the ship’s stern; and so far there would seem to be no advantage in one direction over the other. But, in the first case, the topsides near the compass have weak magnetism; in the second case, they are strongly magnetic: the first position seems therefore preferable.

“In an iron steam-ship, built head to the south, the attraction due to machinery is added to that of the hull, whereas in one built head to the north, the attractive forces of hull and machinery are, in the northern hemisphere, antagonistic, and a position of small, or no ‘semicircular’ deviation for the compass may generally be obtained. To iron steam-vessels engaged on the home or foreign trades in the northern hemisphere, this direction of build is therefore to be preferred.”

And, again, at p. 355, he remarks:

“As every piece of iron not composing a part of, and hammered in the fabrication of the hull,—such as the rudder, funnel, boilers, and machinery, tanks, cooking galleys, fastenings of deck houses, &c.,—are all of a magnetic character differing from the hull of a ship, their proximity should be avoided, and, so far as possible, the compass should be placed so that they may act as correctors of the general magnetism of the hull.

“A compass placed out of the middle line of the deck is affected by the nearest topside, and its deviations must necessarily be much increased if that topside has the dominant polarity, as in ships built east or west.”