Our favourite newspaper has just stated that that gallant ship “is now at Greenhithe waiting to have her compasses adjusted.” So, then, the instruments so accurate at the Observatory a few days ago, are all wrong again on shipboard. Just so. The moment they get to their places afloat, their fidelity to the north wavers,—in one ship more, in another less; but in all in a greater or smaller degree in proportion to the quantity of iron used in the construction of the vessel, and the nearness of that metal to the compasses; in proportion to the number of the iron guns and the total weight of metal carried; to the length of the funnel in steamships, and to the condition of that funnel whether upright or hauled down. All this is both new and strange enough. We have learnt already what loss of ships convoyed and ships wrecked has arisen from these deviations: deviations long neglected on board all vessels and to this hour unrecognised or unattended to in our mercantile marine! Since the Royal Navy, however, has a scientific officer, Captain Johnson, especially employed in attending to the important duty of adjusting the compasses: let us go with him and his assistant, Mr. Brunton, from the Compass Observatory to the anchorage at Greenhithe, and see how he will “swing” the gallant line of battle ship, the Unutterable. The trip occupies a very short time, for we have steam at command. Arrived in the Reach, we find five floating buoys anchored in the stream, one forming a centre, and four being disposed at equal distances about it, just as the five pips are placed upon a card—say the five of spades. The good ship to be operated upon is already fast by the head to the centre buoy, and Captain Johnson having mounted her deck, and his assistant, Mr. Brunton, having been rowed ashore, a rope is run out from the ship’s stern and made fast to one of the corner buoys. The Standard Compass being fixed in the proper position which it is to occupy in the ship, neither too high nor too low, and the guns and other iron being round about it, as they are to remain during the voyage, the mooring ropes are adjusted, and the ship’s head is put due north. Meanwhile, Mr. Brunton has set up a compass ashore, and all being ready, Captain Johnson, at a given moment, observes the bearing of a distant object—the Tower at Shooter’s Hill—noting the bearing of the needle on board. At that instant the pennant that floated at the mast-head is hauled down from the truck. This being the concerted signal, at the same second of time the assistant ashore observed the needle of his compass. The two instruments vary, and the deviation of that on board, compared with that ashore, is due to the iron of the ship. The stern ropes are hauled from one buoy to another, and again made fast, the ship’s head now pointing in another direction. The observations and the signals are repeated. Each deviation of the ship’s compass is carefully noted upon a card previously prepared for the purpose.