The loss of British ships by wreck has been stated at between five and six hundred in a year—or about “a ship and a half-a-day.” This terrible loss has been ascribed to many causes—to the tides and currents of the ocean; to imperfect logs; inaccurate charts; unsteady steerage; inattention to the lead; stress of weather; defective ships, and defective management; but last, if not greatest, says Captain Johnson, who gives this catalogue of sources of disaster, we have the errors of the compass. These errors were noticed—now nearly a couple of centuries ago, and from those days to the present time careful mariners have often called attention to the subject. “Officers in charge of convoys during the war,” continues Captain Johnson, “will probably remember the care with which the general signal was displayed at sunset, to steer a given course during the night,” with what alacrity that signal was repeated by the ships of war in their stations, and answered by every merchant-vessel in the fleet; and they will also possibly remember with what surprise,—nay, indignation,—they observed when daylight came, almost the entire convoy dispersed over the ocean as far as the eye could reach, and mayhap a suspicious looking stranger or two escorting those farthest away, further astray, in despite of all the shots fired during a morning watch to recall them. That such dispersements were in part attributable to the differences of the compasses in each ship, there can be no doubt; but the greatest delinquents in this particular, in all probability, were not the merchant vessels, but rather the ships of war; _the attractive power of their guns upon the compasses_ being now a well-known and constantly proved fact. The Apollo frigate, and forty merchantmen of her convoy, in 1803 were wrecked together on the coast of Portugal, when they believed themselves to be two hundred miles to the westward. The error of the frigate’s compasses is believed to have been the cause of the disaster; and a similar belief exists with respect to the dreadful wrecks of our line of battle ships on the coasts of Jutland and Holland in 1811. The wreck of the Reliance, Indiaman, on the coast of France, when one hundred and nine lives were lost, in 1842, is another painful accident ascribed to errors of the compasses induced by the presence on board of a large iron tank forty-six feet long, the attraction of which had been overlooked—for a hollow tank has a magnetic influence as great as a solid mass of the same external dimensions—and such a mass would weigh four hundred and sixty-eight tons.