“Well, if he is satisfied, I suppose you are,” said Thorndyke, with a smile.  “You are getting a seaside holiday, and being paid for it.  But I didn’t know you were as near to the sea as this.”

We were entering, as he spoke, an artificial gap-way cut through the low cliff, forming a steep cart-track down to the shore.  It was locally known as Sundersley Gap, and was used principally, when used at all, by the farmers’ carts which came down to gather seaweed after a gale.

“What a magnificent stretch of sand!” continued Thorndyke, as we reached the bottom, and stood looking out seaward across the deserted beach.  “There is something very majestic and solemn in a great expanse of sandy shore when the tide is out, and I know of nothing which is capable of conveying the impression of solitude so completely.  The smooth, unbroken surface not only displays itself untenanted for the moment, but it offers convincing testimony that it has lain thus undisturbed through a considerable lapse of time.  Here, for instance, we have clear evidence that for several days only two pairs of feet besides our own have trodden this gap.”

“How do you arrive at the ’several days’?” I asked.

“In the simplest manner possible,” he replied.  “The moon is now in the third quarter, and the tides are consequently neap-tides.  You can see quite plainly the two lines of seaweed and jetsam which indicate the high-water marks of the spring-tides and the neap-tides respectively.  The strip of comparatively dry sand between them, over which the water has not risen for several days, is, as you see, marked by only two sets of footprints, and those footprints will not be completely obliterated by the sea until the next spring-tide—­nearly a week from to-day.”

“Yes, I see now, and the thing appears obvious enough when one has heard the explanation.  But it is really rather odd that no one should have passed through this gap for days, and then that four persons should have come here within quite a short interval of one another.”

“What makes you think they have done so?” Thorndyke asked.

“Well,” I replied, “both of these sets of footprints appear to be quite fresh, and to have been made about the same time.”

“Not at the same time, Jervis,” rejoined Thorndyke.  “There is certainly an interval of several hours between them, though precisely how many hours we cannot judge, since there has been so little wind lately to disturb them; but the fisherman unquestionably passed here not more than three hours ago, and I should say probably within an hour; whereas the other man—­who seems to have come up from a boat to fetch something of considerable weight—­returned through the gap certainly not less, and probably more, than four hours ago.”

I gazed at my friend in blank astonishment, for these events befell in the days before I had joined him as his assistant, and his special knowledge and powers of inference were not then fully appreciated by me.