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.