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.