The last peculiarity which it seems necessary here to mention, was the provision of some kind of instrument for counteracting the effect of the local attraction of so large a mass of iron upon the compasses, and for correcting the errors occasioned thereby. This difficulty had been seriously felt by Colonel Chesney, on board the small iron steamers which he had under his orders, during his expedition to the Euphrates; although he was of opinion, that the placing of the compasses at a certain height _above_ the vessel, so as to be further removed from the sphere of the local attraction of the iron, was sufficient to reduce their errors materially. Without entering into the merits of Barlow's counteracting plates, or Professor Airy's interesting discoveries, it will be sufficient here to mention, that the Nemesis was fitted with correctors, very much according to the system of Professor Airy, but not under his own superintendence; that the experiments were conducted at Liverpool under every disadvantage, and that the result was never perfectly satisfactory. Indeed, the accident which shortly befel her, has been attributed, upon strong grounds, principally to the imperfection of her compasses. It is right, however, to mention, that other vessels, such as the Phlegethon and Pluto, which have been fitted with Airy's correctors, tested according to the most approved principles, and after experiments conducted with great attention, have been totally relieved from this source of danger and anxiety, and have been navigated with perfect accuracy and confidence. We may now come to the interesting moment of the departure of the Nemesis from Liverpool, where she was built. Everything seemed at first to prosper; the weather was favourable, and the machinery perfect in all its parts. She had cleared the narrowest part of the Irish Channel, had passed the coast of Wales, and crossed the entrance to the Bristol Channel; and the course she had been steering would have taken her well clear of the Land's End. It was now the second day since her departure. About two o'clock in the morning, the weather being still hazy and the night dark, she struck heavily on a rock. Of course the engines were instantly stopped, but the _way_ she already had on her appeared sufficient to carry her over the reef; and, indeed, the actual rocks themselves could be seen outside of her, so that she had evidently passed between them and the land, and had merely struck the edge of the reef.