They were protected by the batteries of Callao and of San Lorenzo, a little island in the bay, mounting in all one hundred and sixty guns, in addition to those on board the Spanish war-ships, which consisted of a few frigates, two brigs, and eight gun-boats. To attack such a force lying under the guns of their batteries was too perilous an enterprise for even Cochrane to undertake. He determined, however, to make an attempt to do them at least some damage. He knew that two American men-of-war were expected to arrive in Callao, and he made some slight changes in his flag-ship and the _Lautaro_, hoisted the American flag, and sailed toward the harbour. The Carnival was at the time being carried on, and there was the less chance that a vigilant watch would be kept up by the forts and ships. Unfortunately a dense fog came on, and for eight days the Chilians were forced to remain inactive. At the end of that time, hearing a heavy firing, and believing that one of his ships must have gone into the bay and had been attacked by the enemy, Cochrane stood in the direction of the sound. The other ships did the same, when the fog clearing up in a moment, they discovered each other, and found a small gunboat lying close to them. This they captured at once, and learned that the firing was a salute in honour of the viceroy, who had been paying a visit to the batteries, and was now returning to the town in a brig-of-war which was crowding all sail. The fog again closed in, and hoping the capture of the gun-boat had not been noticed, Lord Cochrane determined to risk a partial engagement rather than withdraw without firing a shot, as a retreat now would raise the spirits of the Spaniards and depress those of the Chilians. Accordingly the _O’Higgins_ and the _Lautaro_ stood on, each having a boat ahead sounding. The wind fell very light, and instead of going in and engaging the Spanish ships as he intended, Lord Cochrane was forced to anchor at some distance from them. The moment that the two Chilian vessels were made out through the fog the ships and batteries opened fire upon them, showing that the capture of the gun-boat had been observed and the sailors and garrison called to the guns.