On the moment the sound of rowing ceased, and from the dense mist just below them there came a sound which was quite unmistakable, namely, the "plop" of something heavy dropped into the water.

That sound, by some remote form of association, suddenly recalled to Michael's mind certain questions Aunt Barbara had asked him about the Emperor's stay at Ashbridge, and his own recollection of his having gone up and down the river in a launch.

There was something further, which he did not immediately recollect.

Yes, it was the request that if when he was here at Christmas he found strangers hanging about the deep-water reach, of which the chart was known only to the Admiralty, he should let her know.

Here at this moment they were overlooking the mist-swathed water, and here at this moment, unseen, was a boat rowing stealthily, stopping, and, perhaps, making soundings.
He laid his hand on Francis's arm with a gesture for silence, then, invisible below, someone said, "Fifteen fathoms," and again the oars creaked audibly in the rowlocks.
Michael took a step towards his cousin, so that he could whisper to him.
"Come back to the boat," he said.