Presents received, May 13, 1875

The question of the position of the standard compass on board ship, whether of wood or iron, is one of the greatest importance with respect to safe navigation. In H. M. ships it is one of the principal duties of the Superintendent of Compasses to secure the best possible position for this compass, and when that position has been determined, to ascertain the horizontal and vertical components of the total magnetic force resulting from the iron used in the construction and equipment of every class of vessel, in order that the correctness of this choice of position may be insured and the facts recorded. With proper instruments in experienced hands these observations can be readily made; but in iron ships it is a somewhat intricate matter to eliminate the part which the hull plays in producing these forces, from those caused by iron used in equipment, unless observations are made on completion of the hull and afterwards as the equipment progresses. In wooden ships, however, the case is different; for the great mass of the ship being of wood and the iron in detached masses, the latter can be attacked in detail, and the disturbance they cause to the several compasses nearly observed.