Davies scanned it with gusto for a moment, climbed to a point of vantage on the boom, and swept his glasses to and fro along the course of the channel.
“Fairly well boomed,” he said, meditatively, “but one or two are very much out. By Jove! that’s a tricky bend there.” He took a bearing with the compass, made a note or two, and sprang with a vigorous leap down on to the sand.
This, I may say, was the only way of “going ashore” that he really liked. We raced off as fast as our clumsy sea-boots would let us, and followed up the course of our channel to the west, reconnoitring the road we should have to follow when the tide rose.
“The only way to learn a place like this,” he shouted, “is to see it at low water. The banks are dry then, and the channels are plain. Look at that boom”—he stopped and pointed contemptuously—“it’s all out of place. I suppose the channel’s shifted there. It’s just at an important bend too. If you took it as a guide when the water was up you’d run aground.”
“Which would be very useful,” I observed.
“Oh, hang it!” he laughed, “we’re exploring. I want to be able to run through this channel without a mistake. We will, next time.” He stopped, and plied compass and notebook. Then we raced on till the next halt was called.
“Look,” he said, “the channel’s getting deeper, it was nearly dry a moment ago; see the current in it now? That’s the flood tide coming up—from the west, mind you; that is, from the Weser side. That shows we’re past the watershed.”
“Watershed?” I repeated, blankly.
“Yes, that’s what I call it. You see, a big sand such as this is like a range of hills dividing two plains, it’s never dead flat though it looks it; there’s always one point, one ridge, rather, where it’s highest. Now a channel cutting right through the sand is, of course, always at its shallowest when it’s crossing this ridge; at low water it’s generally dry there, and it gradually deepens as it gets nearer to the sea on either side. Now at high tide, when the whole sand is covered, the water can travel where it likes; but directly the ebb sets in the water falls away on either side the ridge and the channel becomes two rivers flowing in opposite directions from the centre, or watershed, as I call it. So, also, when the ebb has run out and the flood begins, the channel is fed by two currents flowing to the centre and meeting in the middle. Here the Elbe and the Weser are our two feeders. Now this current here is going eastwards; we know by the time of day that the tide’s rising, therefore the watershed is between us and the yacht.”
“Why is it so important to know that?”