XIII. Account of experiments on iron-built ships, instituted for the purpose of discovering a correction for the deviation of the compass produced by the iron of the ships

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Section I.—Preliminary. In the months of October and November 1835, a series of observations was made under the direction of the Board of Admiralty, by Commander Johnson, R. N., on the iron steam-ship Garry Owen, for ascertaining the amount of disturbance of the compass produced by the magnetic attraction of the iron, of which the ship’s sides and bottom are composed. The details of these experiments are published in the Philosophical Transactions for 1836. Results of great importance were obtained as to the amount of deviation of the compass in different parts of the ship ; and several remarkable experiments were described, which seemed to prove that the ship acted upon external compasses, in the manner of a permanent magnet. But no attempt was made to discover the laws of the magnetic disturbance, or to ascertain its causes; and no attempt could therefore be made to neutralize the ship’s disturbing force by the introduction of new disturbing forces. The last-mentioned point was, however, kept in sight by the Board of Admiralty, and partial arrangements were made for conducting a series of experiments referring expressly to this subject, whenever a favourable opportunity should occur. In the month of July 1838 the iron-built steam-ship the Rainbow was placed by the General Steam Navigation Company at the service of the Admiralty for magnetic examina­tion. The conduct of the experiments was entrusted by the Board to me, and the vessel was immediately placed in the Basin of the Deptford Dock Yard.

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