The Classic Mystery Collection - Arthur Conan Doyle [1007]
'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?'
'Because these currents are strong, and you want to know when you'll lose a fair one and strike a foul one. Besides, the ridge is the critical point when you're crossing on a falling tide, and you want to know when you're past it.'
We pushed on till our path was barred by a big lagoon. It looked far more imposing than the channel; but Davies, after a rapid scrutiny, treated it to a grunt of contempt.
'It's a _cul de sac_,' he said. ' See that hump of sand it's making for, beyond?'
'It's boomed,' I remonstrated, pointing to a decrepit stem drooping over the bank, and shaking a palsied finger at the imposture.
'Yes, that's just where one goes wrong, it's an old cut that's silted up. That boom's a fraud; there's no time to go farther, the flood's making fast. I'll just take bearings of what we can see.'
The false lagoon was the first of several that began to be visible in the west, swelling and joining hands over the ribs of sand that divided them. All the time the distant hissing grew nearer and louder, and a deep, thunderous note began to sound beneath it. We turned our backs to the wind and hastened back towards the Dulcibella, the stream in our channel hurrying and rising alongside of us.
'There's just time to do the other side,' said Davies, when we reached her, and I was congratulating myself on having regained our base without finding our communications cut. And away we scurried in the direction we had come that morning, splashing through pools and jumping the infant runnels that were stealing out through rifts from the mother-channel as the tide rose. Our observations completed, back we travelled, making a wide circuit over higher ground to avoid the encroaching flood, and wading shin-deep in the final approach to the yacht.
As I scrambled thankfully aboard, I seemed to hear a far-off voice saying, in languid depreciation of yachting, that it did not give one enough exercise. It was mine, centuries ago, in another life. From east and west two sheets of water had overspread the desert, each pushing out tongues of surf that met and fused.
I waited on deck and watched the death-throes of the suffocating sands under the relentless onset of the sea. The last strongholds were battered, stormed,