When at last Monk and Rupert were ready to sail they had to wait for a favourable wind and tide, and, with the help of their pilots, solve a somewhat delicate problem. This problem was something like that which a general on land has to solve when it is a question of moving a large force through defiles of which the other end is watched by the enemy's main army. But it had special complications that the soldier would not have to take into account. Monk's fleet sailing in line ahead, the only order in which it could traverse the narrow channels, would cover about nine miles from van to rear. There were then no accurate charts of the Thames estuary such as we now possess, and the pilots of the time believed the possible ways out for large ships to be fewer and more restricted than we know them to be at present. They advised Monk to take his fleet out from the Nore through the Warp and the West Swin, which form a continuous, fairly deep channel on the Essex side of the estuary along the outer edge of the Maplin Sands. At the outer end of the Maplins a long, narrow sandbank, known as the Middle Ground, with only a few feet of water over it at low tide, divides the channel into two parallel branches, the East Swin and the Middle Deep. At the end of the Middle Ground these two channels and a third (known as the Barrow Deep) unite to form the broad King's Channel (also known as the East Swin), where there is plenty of sea room, and presently this again expands into the open sea. In those old days of sailing-ships a fleet working its way out of the narrower channels inside the Middle Deep in presence of an enemy would court destruction if the whole of its fighting strength could not be brought out into the wide waters of the King's Channel on a single tide. If only part of it got out before the tide turned, the van might be destroyed during the long hours of waiting for the rearward ships to get out and join in.