The Revue Maritime el Coloniale recently published a letter on the deviations to which the needle is liable in consequence of the substitution of iron for wood in ships.

One of the latest contrivances for diminishing this serious inconvenience is the correcting compass, which affords the means of taking the sun's position, whereby the deviation may be corrected. It has sometimes been supposed that fogs and certain other states of the atmosphere could influence the needle; but this has not been borne out by observation.

Lightning alone exercises a decided influence on the needle by reversing its points, so that north becomes south, and conversely. When a vessel is nearing land, the needle is said to be affected; and certain rocks there are that exercise a decided magnetic influence on the compass, volcanic rocks especially, but this influence is not felt on board ships. But the action of the iron forming the ship's sides is far different; nothing, not even the interposition of a thick non-magnetic body, will stop its influence; far less, as some have believed, a copper coating or thick paint. But the real danger proceeds from another source, since the ship herself, under her weight of canvas, may increase the deviation of the needle. From experiments made on board an iron-built sailing vessel, provided with iron rigging and lower yards of steel and with two binnacle compasses on her poops, and a third placed between the mizen and mainmasts, the lower part of which was all of iron, the deviations of the needle were respectively 56°, 24°, and 35°. Without entering into further details on this matter, the writer of the article concludes with condemning the imprudence of those who freight an iron vessel before she has been at sea for a considerable time, in order to ascertain how her compass behaves. Moreover, a captain undertaking the command of an iron ship should be called upon to show that he has previously been on board such a vessel on a long voyage, so that he may know how to deal with the deviations observable on board the vessel to be commanded.