LOCAL ATTRACTION, a term accepted by men of science as applicable to influences exerted on the compass by substances ou ship board. Few subjects have taxed the patience and intelligence of nautical men so much u that of the disturbance of the compass by local attraction i • nor has the consideration of the subject by living philosophers been productive of more than a verification of certain suspected facts.
Professor Airy has indeed deeply investigated the causes as they ordinarily appear, and has been successful in providing some alleviation of the evils attendant upon a disturbed compass; but in other respects nothing of permanent benefit has scented to the compass question either from committees, or from elaborate experiment; nothing which aids the seaman by an infallible remedy. Were a ship to remain always in nearly the same part of the globe, and her local attraction to remain a constant quantity, Airy's improvements would be all that could be &wired; as it is, no other mechanical means of correcting by magnets in a long voyage is so convenient or so manageable.
1 hat the compeers was subject to changes and deflection. irrespective of terrestrial magnetism, was of little importance in ships constructed only of wood. irregularities of the compass were occasionally detected, but these were mostly attributable to iron bolts or nails, or accidental deposits of metal, such as a knife or marlinspike upon or near the binnacle. Beyond these casual or accidental disturbances, the mariner of olden time had little to do with local attraction. An iron cargo occasionally caused him some extra watehfulnhss, but the error once aseertaine41 left him at ease upon the subject of his compass. No sooner however had the interest of merchants increased the space for stowage in their ships, and, in adopting modern improvements, introduced iron knees, chain rigging, and various other metallic appliances, than the consideration of the actual compass course to be steered became involved in new and perplexing corrections, and various speculations concerning a ehip'a condition as altered by these improve ments became the subject of study. For nearly a quarter of a century title state of things was suffered to progress, when the establishment of the present Board of 'and the intelligence of its naval heads, introduced a new and still scrutiny.
Ships had begun to be constructed of iron only ; some had been built many years previously—for the writer the first iron vessel which ever anchored in the Downs, a fine brig called the ' Tontine; in about the year 1816. From iron hulls, iron masts and even yards, soon wereto bo found afloat in British waters. And as if the compass were to be threatened with total rejection, iron ships were soon fitted with steam engines. 'Wooden steatners had already (bout thirty years since) exercised the patience and vigilance of ship masters to a large extent, but changes so enormous were gradually producing a malignant phantom more and more forbidding In its aspect, from its power of eluding the grasp of its pursuers, and deluding them into still deeper swamps of error and disappointment. Such effects then had at first " local attraction." Some, to avert the evils of an invisible and fickle agency, fastened magnets to their decks as palliative., but they did Do with as many misgivings as they still fasten the horse shoe to the bowsprit end. Any species of " charm " which the credulity of the sup erstitioue could invoke to avert mischief from the compass was eagerly applied to its bowl, or the binnacle, or to the sleek, until in 1654, the unfortunate catastrophe of the Tayleur: a fine chip of above 8000 tons (actual measurement) built of iron throughout, and having compartmental also of iron, was lost with 2110 of her crew at Larntay, on the Irish coast The Investigation of the causes of her loss resulted In an official report from Captain Walker, (one of the naval heads of the Board of Trade) attributtng her destruction to the " supposition" on the part of the Captain,' that the wove.s before the helmsman was correct," and alleging that the captain " was aware there was a difference of two points between the cetnpasaes, but he knew not which was in error." We see then the cffucts of compass disturbance, and proceed to !menthe the causes, hut have to"deal with the circumstance of local ItIractIon being liable to sudden and great changes, so that corrections Which are available and effective at one period are totally useless at mother. It is this change which baffles us, and in it lies the greatest amount of danger to navigation.