human
I suppose you steer by the compass, Master Chambers? Geoffrey said.
gpt
Which is the compass? I have heard about it, always pointing to the north.
human
It's down below, young sir; I will show it you presently. We steer by that at night, or when it's foggy; but on a fine day like this there is no need for it. There are marks put up on all the sands, and we steer by them. You see, the way the wind is now we can lay our course for the Whittaker. That's a cruel sand, that is, and stretches out a long way from a point lying away on the right there. Once past that we bear away to the south-west, for we are then, so to speak, fairly in the course of the river. There is many a ship has been cast away on the Whittaker. Not that it is worse than other sands. There are scores of them lying in the mouth of the river, and if it wasn't for the marks there would be no sailing in or out.
gpt
Who put up the marks?
human
They are put up by men who make a business of it. There is one boat of them sails backwards and forwards where the river begins to narrow above Sheerness, and every ship that goes up or down pays them something according to her size. Others cruise about with long poles, putting them in the sands wherever one gets washed away. They have got different marks on them. A single cross-piece, or two cross-pieces, or a circle, or a diamond; so that each sand has got its own particular mark. These are known to the masters of all ships that go up and down the river, and so they can tell exactly where they are, and what course to take. At night they anchor, for there would be no possibility of finding the way up or down in the dark. I have heard tell from mariners who have sailed abroad that there ain't a place anywhere with such dangerous sands as those we have got here at the mouth of the Thames.