It must be some place where there were several staircases, and one marked out from the others by having thirty-nine steps.
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.