Then I had a sudden thought, and hunted up all the steamer sailings. There was no boat which left for the Continent at 10:17 p.m.

Why was high tide so important? If it was a harbour it must be some little place where the tide mattered, or else it was a heavy-draught boat. But there was no regular steamer sailing at that hour, and somehow I didn’t think they would travel by a big boat from a regular harbour. So it must be some little harbour where the tide was important, or perhaps no harbour at all.

But if it was a little port I couldn’t see what the steps signified. There were no sets of staircases on any harbour that I had ever seen. It must be some place which a particular staircase identified, and where the tide was full at 10:17. On the whole it seemed to me that the place must be a bit of open coast. But the staircases kept puzzling me.

Then I went back to wider considerations. Whereabouts would a man be likely to leave for Germany, a man in a hurry, who wanted a speedy and a secret passage? Not from any of the big harbours. And not from the Channel or the West Coast or Scotland, for, remember, he was starting from London. I measured the distance on the map, and tried to put myself in the enemy’s shoes. I should try for Ostend or Antwerp or Rotterdam, and I should sail from somewhere on the East Coast between Cromer and Dover.

All this was very loose guessing, and I don’t pretend it was ingenious or scientific. I wasn’t any kind of Sherlock Holmes. But I have always fancied I had a kind of instinct about questions like this. I don’t know if I can explain myself, but I used to use my brains as far as they went, and after they came to a blank wall I guessed, and I usually found my guesses pretty right.

So I set out all my conclusions on a bit of Admiralty paper. They ran like this: