Among the causes which thus operate, we may name the fancy rails leading to state-cabins and saloons. These, beneath a highly-polished covering of brass, often conceal many hundredweights of iron. Cabin stoves and funnels, immediately under and alongside the compass, are frequently unsuspected. A noble transport, during the late war, carrying troops and stores, pursued her course by day with unswerving fidelity, but at night the compass was as wild as the waves themselves. After diligent search it was found that the brazier, in preparing the binnacle lamps, had introduced a concealed iron-wire hoop to strengthen their frame-work. The stowage of iron in cargo does not receive the attention it deserves, and we consider it should be imperative for every vessel which carries it to be swung for the local deviation before quitting port, and a certificate duly lodged before clearing the Customs. When the _Agamemnon_ adjusted compasses preparatory to sailing upon the last unsuccessful expedition to lay the Atlantic cable, it was discovered that the presence of the enormous coil in her hold caused a deviation of no less than seventeen degrees! Had she been a merchant ship, no similar verification would have been made, and the sign-post which showed the path upon the trackless waters would only have pointed to mislead. It is remarkable how much misapprehension on the nature of magnetic action exists even among men of high intelligence. A competent witness, in a recent law-trial, in a case of wreck, arising chiefly from a want of knowledge of the laws of magnetism in the navigation of the ship, stated that seamen in general believed, that if a cargo of iron was covered over, its effects were cut off from the compass. A leading counsel in the case sympathized with the general ignorance, because he confessed that he shared it. The adjustment of compasses by magnets is a most delicate operation, and has received much attention from some of our leading men in science. An able committee, under the auspices of the Board of Trade, are now engaged, in the midst of an iron navy, in the port of Liverpool, in elucidating the whole of the subject. We feel bound, however, to record our opinion against the indiscriminate employment of all the nostrums prescribed by the compass-doctors or quacks at many of our seaports. Let the shipowner consult such reports of the Liverpool Committee as have been already published, or follow the Admiralty plan of having at least one good compass in a position free from all magnetic influences. In some of the large ocean steamers a standard compass is fitted high up in the mizen mast, and we hear that it is proposed to build a special stage on board the _Great Eastern_, in order to keep the compass from being affected by the immense body of iron in her fabric.